Background
The global population aged 60 years or over has expanded rapidly in recent years, reaching 901 million in 2015 [
1]. By 2050, it is projected to reach 2.1 billion, accounting for at least 20% of the total population [
1]. In China, it also remains a daunting health challenge. According to the sixth Chinese national population census in 2010 [
2], the proportion of elderly individuals aged 60 years or over reached 13.3% and it is projected to increase to 19.3% by 2025. Aging is associated with a decline in cognitive functioning and social relationships [
3], physical impairment [
4], and a majority of chronic diseases [
5], and, thus, results in poor quality of life and strong dependency on others. Moreover, life expectancy at birth had been projected to rise from 69 years in 1990 to 75 years in 2013 [
6]. Living longer may lead to more age-related problems.
Successful aging, which is regarded as a “gold standard” for assessing the health conditions of older people [
7], implies aging in adults associated with no diseases, high physical and cognitive functioning, and active social engagement [
8,
9]. The prevalence of successful aging among older adults reported by previous studies in several countries ranged from 11.9 to 64.6% [
10‐
12]. In China, the proportion is relatively low, reaching only 13.3% [
13]. The low prevalence of successful aging warrants more attention in this country.
Daytime napping is a widely adopted behavior in many countries, and the prevalence of daytime napping increases with age [
14]. In China, daytime napping is common among older adults. Studies found that 68.6% of the older Chinese exhibited habitual daytime napping [
15], with an average nap duration of 52.6 mins [
16]. Although daytime napping is popular in older adults, its overall effect on health remains unclear. Some studies revealed that daytime napping was associated with high cognitive functioning [
17] and positive mood [
18], while others indicated that it was associated with chronic diseases [
19], depression [
20], and physical impairment [
21]. Therefore, it is necessary to thoroughly evaluate the association between daytime napping and health among older adults. Considering the aforementioned significance of successful aging in older adults and the comprehensive multidimensional constructs successful aging has covered, the primary objective of the study was to assess the association between daytime napping and successful aging among China’s older adults using a large nationally representative sample.
In addition, previous studies have shown significant association between nighttime sleep and successful aging [
13] and have demonstrated combined effects of daytime napping and nighttime sleep on certain diseases [
22,
23]. Thus, the secondary objective for the study was to examine differences in the association between daytime napping and successful aging in different nighttime sleep duration groups.
Results
Table
1 showed the characteristics of participants with different nighttime sleep durations. Overall, the proportion of males was 50.0% among all participants (
n = 3734). The mean age of the participants was 67.95 years and 1227 participants (16.4%) were aged 75 years or above.
Table 1Characteristics of participants with different nighttime duration
Successful aging |
No | 6191 (86.1) | 2156 (87.9) | 2125 (83.0) | 1910 (87.7) |
Yes | 999 (13.9) | 296 (12.1) | 435 (17.0) | 268 (12.3) |
Low probability of disease |
No | 3822 (51.2) | 1363 (53.3) | 1359 (51.5) | 1100 (48.4) |
Yes | 3647 (48.8) | 1195 (46.7) | 1281 (48.5) | 1171 (51.6) |
No disease-related disability |
No | 566 (7.6) | 262 (10.3) | 159 (6.0) | 145 (6.4) |
Yes | 6884 (92.4) | 2289 (89.7) | 2475 (94.0) | 2120 (93.6) |
High cognitive functioning |
No | 3303 (45.2) | 1253 (50.2) | 940 (36.2) | 1110 (50.2) |
Yes | 4000 (54.8) | 1243 (49.8) | 1657 (63.8) | 1100 (49.8) |
High physical functioning |
No | 1480 (20.1) | 652 (25.9) | 413 (15.9) | 415 (18.5) |
Yes | 5883 (79.9) | 1866 (74.1) | 2190 (84.1) | 1827 (81.5) |
Active engagement with life |
No | 3660 (49.0) | 1277 (49.9) | 1198 (45.4) | 1185 (52.2) |
Yes | 3809 (51.0) | 1281 (50.1) | 1442 (54.6) | 1086 (47.8) |
Daytime napping (per day) |
0 min | 3012 (40.5) | 1220 (47.9) | 968 (36.8) | 824 (36.6) |
1–60 min | 2928 (39.4) | 926 (36.4) | 1149 (43.6) | 853 (37.9) |
> 60 min | 1493 (20.1) | 401 (15.7) | 517 (19.6) | 575 (25.5) |
Sex |
Male | 3734 (50.0) | 1080 (42.2) | 1436 (54.4) | 1218 (53.6) |
Female | 3735 (50.0) | 1478 (57.8) | 1204 (45.6) | 1053 (46.4) |
Age |
60–64 | 2791 (37.4) | 870 (34.0) | 1093 (41.4) | 828 (36.5) |
65–74 | 3451 (46.2) | 1212 (47.4) | 1211 (45.9) | 1028 (45.3) |
≥ 75 | 1227 (16.4) | 476 (18.6) | 336 (12.7) | 415 (18.3) |
Community type |
Rural | 5588 (76.0) | 1940 (77.1) | 1819 (70.3) | 1829 (81.5) |
Urban | 1761 (24.0) | 577 (22.9) | 770 (29.7) | 414 (18.5) |
Education |
No formal education | 2428 (33.3) | 940 (37.5) | 702 (27.4) | 786 (35.5) |
Did not finish primary school | 1574 (21.6) | 541 (21.6) | 545 (21.2) | 488 (22.1) |
Sishu/Homeschool/Elementary school | 1745 (24.0) | 553 (22.1) | 658 (25.6) | 534 (24.2) |
Middle school and above | 1535 (21.1) | 471 (18.8) | 661 (25.8) | 403 (18.2) |
Marital status |
Cohabited (married or not) | 5823 (78.0) | 1880 (73.5) | 2159 (81.8) | 1784 (78.6) |
Did not cohabit (Separated/divorced/widowed and never married) | 1646 (22.0) | 678 (26.5) | 481 (18.2) | 487 (21.4) |
Annual household income |
≤ 2000 CNY | 2474 (33.1) | 917 (35.8) | 777 (29.4) | 780 (34.3) |
2001–10,000 CNY | 1796 (24.1) | 657 (25.7) | 591 (22.4) | 548 (24.1) |
10,001–25,000 CNY | 1485 (19.9) | 483 (18.9) | 565 (21.4) | 437 (19.2) |
> 25,000 CNY | 1714 (22.9) | 501 (19.6) | 707 (26.8) | 506 (22.3) |
Tobacco use |
No | 3942 (52.8) | 1466 (57.4) | 1325 (50.2) | 1151 (50.7) |
Yes | 3523 (47.2) | 1090 (42.6) | 1314 (49.8) | 1119 (49.3) |
Alcohol consumption |
No | 5001 (67.0) | 1759 (68.8) | 1705 (64.6) | 1537 (67.7) |
< once a month | 550 (7.4) | 187 (7.3) | 218 (8.3) | 145 (6.4) |
≥ once a month | 1915 (25.6) | 611 (23.9) | 716 (27.1) | 588 (25.9) |
Physical activity |
No | 4280 (57.3) | 1482 (57.9) | 1487 (56.3) | 1311 (57.7) |
Yes | 3189 (42.7) | 1076 (42.1) | 1153 (43.7) | 960 (42.3) |
Low quality of nighttime sleep |
< 1 day/week | 3805 (51.3) | 716 (28.1) | 1511 (57.6) | 1578 (70.1) |
1–2 days/week | 1008 (13.6) | 311 (12.2) | 421 (16.1) | 276 (12.3) |
3–4 days/week | 978 (13.2) | 436 (17.1) | 350 (13.3) | 192 (8.5) |
5–7 days/week | 1626 (21.9) | 1082 (42.5) | 340 (13.0) | 204 (9.1) |
Body Mass Index |
Normal | 3857 (51.6) | 1357 (53.0) | 1333 (50.5) | 1167 (51.4) |
Underweight | 569 (7.7) | 209 (8.2) | 177 (6.7) | 183 (8.1) |
Overweight | 2249 (30.1) | 728 (28.5) | 832 (31.5) | 689 (30.3) |
Obesity | 794 (10.6) | 264 (10.3) | 298 (11.3) | 232 (10.2) |
Nighttime sleep duration (per night) |
< 6 h | 2558 (34.3) | – | – | – |
6 to less than 8 h | 2640 (35.3) | – | – | – |
≥ 8 h | 2271 (30.4) | – | – | – |
Among a total of 7469 respondents, 40.5% reported no daytime napping (0 min/day), 39.4% reported moderate daytime napping (1–60 min/day) and 20.1% reported long daytime napping (> 60 mins/day). The mean duration of daytime napping was 40.8 ± 0.5 mins in all respondents and 68.6 ± 0.6 mins in participants who habitually napped.
The proportion of participants with “successful aging” was 13.9%. Additionally, among the total participants, 48.8, 92.4, 54.8, 79.9, and 51.0% achieved “low probability of disease,” “no disease-related disability,” “high cognitive functioning,” “high physical functioning” and “active engagement with life,” respectively.
Table
2 shows the association between daytime napping and successful aging before and after stratification by nighttime sleep duration after adjusting for possible confounders. Compared with the 0 min/day napping group, the > 60 mins/day napping group was found to be associated with lower odds of achieving successful aging (OR, 0.762; 95% CI, 0.583–0.996). Participants who did not achieve successful aging were more likely to be older in age, less educated, living in rural areas, having a low-income, performing inadequate physical activities and reporting low quality of nighttime sleep. In the subgroup analyses, stratified by nighttime sleep duration, napping more than 60 mins/day was associated with a lower likelihood of achieving successful aging in the long nighttime sleep duration group (≥8 h/night) (OR, 0.617; 95% CI, 0.387–0.984).
Table 2Multivariable Logistic Regression on Daytime Napping and Successful Aging
Daytime napping (per day) |
0 min (ref) | – | – | – | – |
1–60 min | 1.012 (0.829–1.234) | 1. 270 (0.887–1.817) | 1.004 (0.736–1.370) | 0.818 (0.559–1.196) |
> 60 min | 0.762 (0.583–0.996) | 0.875 (0.516–1.484) | 0.837 (0.550–1.276) | 0.617 (0.387–0.984) |
Sex |
Male (ref) | – | – | – | – |
Female | 1.060 (0.804–1.396) | 0.727 (0.433–1.222) | 1.083 (0.706–1.662) | 1.541 (0.922–2.573) |
Age |
60–64(ref) | – | – | – | – |
65–74 | 0.614 (0.507–0.745) | 0.639 (0.447–0.914) | 0.597 (0.443–0.804) | 0.620 (0.429–0.897) |
≥ 75 | 0.451 (0.327–0.621) | 0.344 (0.189–0.625) | 0.468 (0.274–0.798) | 0.559 (0.320–0.978) |
Education |
No formal education (illiterate) (ref) | – | – | – | – |
Did not finish primary school | 2.170 (1.606–2.933) | 1.906 (1.092–3.326) | 1.947 (1.192–3.180) | 2.635 (1.541–4.504) |
Sishu/homeschool/elementary school | 3.053 (2.280–4.087) | 2.937 (1.717–5.022) | 2.875 (1.801–4.589) | 3.439 (2.018–5.861) |
Middle school and above | 4.952 (3.685–6.656) | 5.261 (3.098–8.934) | 4.252 (2.640–6.849) | 5.887 (3.394–10.209) |
Marital status |
Married and cohabited (ref) | – | – | – | – |
Other (Separated/divorced/widowed and never married) | 0.783 (0.612–1.003) | 0.995 (0.663–1.494) | 0.757 (0.501–1.144) | 0.590 (0.358–0.972) |
Community type |
Rural area (ref) | – | – | – | – |
Urban area | 1.271 (1.030–1.568) | 1.591 (1.088–2.328) | 1.070 (0.774–1.480) | 1.391 (0.923–2.094) |
Annual household income |
≤ 2000 CNY (ref) | – | – | – | – |
2000 < x ≤ 10,000 CNY | 1.232 (0.953–1.593) | 1.042 (0.667–1.629) | 1.278 (0.839–1.947) | 1.447 (0.892–2.348) |
10,000 < x ≤ 25,000 CNY | 1.177 (0.901–1.537) | 0.932 (0.572–1.518) | 1.268 (0.832–1.932) | 1.340 (0.803–2.236) |
> 25,000 CNY | 1. 456 (1.135–1.868) | 1.039 (0.649–1.662) | 1.429 (0.960–2.127) | 2.010 (1.274–3.170) |
Tobacco use |
No (ref) | – | – | – | – |
Yes | 1.078 (0.833–1.395) | 0.845 (0.515–1.384) | 1.220 (0.817–1.822) | 1.157 (0.721–1.856) |
Alcohol consumption |
No (ref) | – | – | – | – |
< once a month | 1.350 (1.033–1.764) | 1.819 (1.123–2.948) | 1.033 (0.679–1.571) | 1.559 (0.933–2.606) |
≥ once a month | 0.930 (0.710–1.217) | 0.911 (0.543–1.529) | 0.785 (0.516–1.194) | 1.224 (0.748–2.004) |
Physical activity |
No (ref) | – | – | – | – |
Yes | 1.353 (1.130–1.620) | 1.185 (0.847–1.658) | 1.351 (1.019–1.790) | 1.545 (1.100–2.169) |
Low quality of nighttime sleep |
< 1 day/week (ref) | – | – | – | – |
1–2 days/week | 1.078 (0.834–1.394) | 1.272 (0.758–2.134) | 0.843 (0.570–1.248) | 1.366 (0.854–2.187) |
3–4 days/week | 0.615 (0.452–0.837) | 0.735 (0.433–1.248) | 0.594 (0.374–0.943) | 0.522 (0.245–1.111) |
5–7 days/week | 0.659 (0.501–0.867) | 0.802 (0.531–1.212) | 0.468 (0.277–0.791) | 0.843 (0.429–1.656) |
Nighttime sleep duration (per night) |
6 to less than 8 h (ref) | – | – | – | – |
< 6 h | 0.926 (0.734–1.169) | – | – | – |
≥ 8 h | 0.936 (0.750–1.168) | – | – | – |
The association between daytime napping and the five dimensions of successful aging after adjustment for possible confounders are shown in Table
3.
Table 3Association between daytime napping and five components of successful aging among the total population and stratified by the nighttime sleep duration groups
| Low probability of disease |
TOTAL OR (95%CI) | Nighttime sleep duration (per night) |
< 6 h OR (95%CI) | 6 to less than 8 h OR (95%CI) | ≥8 h OR (95%CI) |
Daytime napping (per day) |
0 min | 1 | 1 | 1 | 1 |
1–60 min | 0.776 (0.686–0.878) | 0.818 (0.666–1.004) | 0.740 (0.599–0.916) | 0.753 (0.598–0.949) |
> 60 min | 0.761 (0.651–0.889) | 0.770 (0.581–1.020) | 0.720 (0.548–0.947) | 0.759 (0.582–0.988) |
| No disease-related disability |
TOTAL OR (95%CI) | Nighttime sleep duration (per night) |
< 6 h OR (95%CI) | 6 to less than 8 h OR (95%CI) | ≥8 h OR (95%CI) |
Daytime napping (per day) |
0 min | 1 | 1 | 1 | 1 |
1–60 min | 0.848 (0.674–1.067) | 0.931 (0.675–1.285) | 0.746 (0.462–1.203) | 0.730 (0.448–1.188) |
> 60 min | 0.595 (0.455–0.777) | 0.859 (0.558–1.324) | 0.355 (0.213–0.591) | 0.581 (0.345–0.980) |
| High cognitive functioning |
TOTAL OR (95%CI) | Nighttime sleep duration (per night) |
< 6 h OR (95%CI) | 6 to less than 8 h OR (95%CI) | ≥8 h OR (95%CI) |
Daytime napping (per day) |
0 min | 1 | 1 | 1 | 1 |
1–60 min | 1.233 (1.063–1.430) | 1.608 (1.257–2.056) | 1.187 (0.918–1.535) | 0.925 (0.699–1.223) |
> 60 min | 1.062 (0.883–1.277) | 1.226 (0.880–1.707) | 0.875 (0.634–1.209) | 1.060 (0.774–1.452) |
| High physical functioning |
TOTAL OR (95%CI) | Nighttime sleep duration (per night) |
< 6 h OR (95%CI) | 6 to less than 8 h OR (95%CI) | ≥8 h OR (95%CI) |
Daytime napping (per day) |
0 min | 1 | 1 | 1 | 1 |
1–60 min | 0.927 (0.794–1.083) | 1.023 (0.808–1.295) | 0.783 (0.585–1.049) | 0.952 (0.704–1.287) |
> 60 min | 0.775 (0.640–0.937) | 0.870 (0.632–1.196) | 0.573 (0.405–0.811) | 0.881 (0.627–1.238) |
| Active engagement with life |
TOTAL OR (95%CI) | Nighttime sleep duration (per night) |
< 6 h OR (95%CI) | 6 to less than 8 h OR (95%CI) | ≥8 h OR (95%CI) |
Daytime napping (per day) |
0 min | 1 | 1 | 1 | 1 |
1–60 min | 1.183 (1.044–1.339) | 1.246 (1.014–1.530) | 1.091 (0.880–1.353) | 1.200 (0.949–1.519) |
> 60 min | 1.120 (0.958–1.310) | 1.223 (0.923–1.619) | 0.880 (0.667–1.159) | 1.300(0.993–1.700) |
Compared with 0 min/day napping group, participants who reported a moderate duration of napping (1–60 min/day) or long duration of napping (> 60 mins/day) had lower odds of “low probability of disease” (OR, 0.776; 95% CI, 0.686–0.878 and OR, 0.761; 95% CI, 0.651–0.889, respectively). This association was significant in the 6 to less than 8 h/night group (OR, 0.740; 95% CI, 0.599–0.916) and the ≥8 h/night group (OR, 0.753; 95% CI, 0.598–0.949), while it was not significant in the < 6 h/night group in subgroup analysis.
In comparison with the 0 min/day napping group, the > 60 min/day napping group was less likely to achieve “no disease-related disability” (OR, 0.595; 95% CI, 0.455–0.777). However, the association was only significant in the 6 to less than 8 h/night group (OR, 0.355; 95% CI, 0.213–0.591) and the ≥8 h/night group (OR, 0.581; 95% CI, 0.345–0.980) in stratified analysis.
Compared with the 0 min/day napping group, those napped 1–60 min/day were more likely to achieve “high cognitive functioning” (OR, 1.233; 95% CI, 1.063–1.430). After stratification, the association between daytime napping and “high cognitive functioning” was only found to be significant in the < 6 h/night group (OR, 1.608; 95% CI, 1.257–2.056).
In comparison with those who had no daytime napping (0 min/day), the > 60 min/day napping group had lower odds of having “high physical functioning” (OR, 0.775; 95% CI, 0.640–0.937). In stratified analysis, napping more than 60 mins/day was negatively associated with achieving “high physical functioning” in the 6 to less than 8 h/night group (OR, 0.573; 95% CI, 0.405–0.811) compared with the 0 min/day napping group.
The 1–60 min/day napping group was more likely to have “active engagement with life,” in comparison with the 0 min/day napping group (OR, 1.183; 95% CI, 1.044–1.339). After stratification, napping 1–60 min/day with < 6 h/night sleep was found to be significantly associated with respondents achieving “active engagement with life” (OR, 1.246; 95% CI, 1.014–1.530).
Discussion
The study examined the association between daytime napping and successful aging among China’s older adults using multivariable logistic regressions and differences in the relationship based on nighttime sleep duration using stratified analysis. Findings suggest that compared with the 0 min/day napping group, the > 60 min/day napping group was found to be associated with lower odds of achieving successful aging, especially in the ≥8 h/night subgroup.
In our study, 59.3% of the older adults reported habitual daytime napping. Of the total participants, 39.2% reported moderate daytime napping duration and 20.1% reported long daytime napping duration. These findings add to current evidence supporting the fact that daytime napping is a widely adopted lifestyle behavior in older adults in China [
16]. Of the total participants, 13.7% achieved “successful aging.” Regarding the five dimensions of successful aging, 48.8, 92.4, 54.8, 79.9 and 51.0% achieved “low probability of disease,” “no disease-related disability,” “high cognitive functioning,” “high physical functioning,” and “active engagement with life,” respectively. This was in accordance with a previous study conducted in 2013 [
13], which reported that 13.3% of older adults in China achieved “successful aging,” and 41.9, 92.6, 54.5, 70.6, and 46.3% achieved “no major disease,” “no disability,” “high cognitive functioning,” “high physical functioning” and “active engagement with life,” respectively.
Our study showed that compared with the 0 min/day napping group, the > 60 min/day napping group was associated with a lower likelihood of successful aging. This was partially consistent with former studies, which demonstrated that long duration of daytime napping (> 60 min/day) was independently associated with major diseases [
28], cognitive impairment [
29], and physical activity deficit [
30]. However, this association was only found to be significant in the ≥8 h/night group. Hence, respondents with long duration of both nighttime sleep (≥8 h/night) and long daytime napping (> 60 min/day) were less likely to achieve successful aging. As indicated by previous studies, both long duration of daytime napping and long duration of nighttime sleep were indicators of sleep disturbance [
22] and both were associated with health issues independently [
13,
31,
32]. Similar results were found regarding the association between daytime napping and stroke in a previous study, which reported that long daytime napping combined with long nighttime sleep duration was significantly associated with a high risk of stroke [
23]. Therefore, our observation of differences in the association between daytime napping and successful aging based on duration of nighttime sleep could be partially corroborated by these findings.
The association between daytime napping and the five dimensions of successful aging varied. Daytime napping was positively associated with “high cognitive functioning” and “active engagement with life,” and was negatively related to “a low probability of disease,” “no disease-related disability,” and “high physical functioning.”
In comparison with the 0 min/day napping group, the napping groups (1–60 min/day, > 60 min/day) had lower odds of achieving “low probability of disease.” This was consistent with former studies that revealed that daytime napping was associated with respiratory [
33], cardiovascular disease [
34], stroke [
23], cancer [
35], depression [
20], diabetes [
36], asthma [
37], hypertension [
38], and obesity [
39]. However, the association was only significant in the 6 to less than 8 h/night and ≥ 8 h/night groups, but not significant in the < 6 h/night group. A previous study demonstrated that long daytime napping and long nighttime sleep duration were jointly associated with diabetes [
40]; this partially validated our findings.
Compared with the 0 min/day napping group, the > 60 min/day napping group had lower odds of having “no disease-related disability.” This could be partially explained by the findings in a previous study, which revealed that excessive daytime napping was associated with a higher limitation of performing instrumental activities of daily living (IADL) [
21]. When stratified by nighttime sleep duration, the associations were only significant in the 6 to less than 8 h/night group and the ≥8 h/night group. This may be because difficulties with performing ADLs might result in impairment of mobility, and people with restricted mobility might have a tendency for excessive daytime napping and long duration of nighttime sleep to tackle the effects of sleep issues (e.g. sleep disturbance, sleep efficiency) [
41].
Previous studies revealed that moderate duration of daytime napping was associated with improved waking cognitive performance [
17] and alertness [
42]. Although a different measure of cognitive functioning was used, our study found that moderate daytime napping (1–60 min/day) was associated with higher cognitive functioning. Moreover, this association was only found in the < 6 h/night group, which implies that participants with a combination of both moderate daytime napping and inadequate nighttime sleep were more likely to have “high cognitive functioning.” This may be explained by a former study [
43], which showed that daytime napping played a compensatory role in the sleep routine.
Compared with those with no daytime napping (0 min/day), the > 60 min/day napping group had lower odds of “high physical functioning.” After stratification, the association was only found in the participants with 6 to less than 8 h/night sleep, but not in the participants with < 6 h/night or ≥ 8 h/night sleep. A previous study found that inadequate sleep and long nighttime sleep duration was associated with physical impairment and low neuromuscular performance [
21]; this was partially supported by our observation that the proportion of participants with “high physical functioning” was comparatively low in the < 6 h/night (74.1%) and ≥ 8 h/night (81.5%) groups, so the effects of daytime napping on physical functioning in these two groups might have been diminished.
Compared with the participants who had 0 min/day napping, the participants in the 1–60 min/day napping group were more likely to have “active engagement with life.” In stratified analysis, this association was significant in those who had < 6 h/night sleep. This could be explained by our observation that the respondents who reported napping of 1–60 min/day were more likely to have a higher cognition in the < 6 h/night sleep group, considering that higher cognition increases the likelihood to actively engage with life [
44].
There were some limitations to this study. First, there was no objective measurement in some variables (e.g. daytime napping, nighttime sleep duration and diagnosed diseases in successful aging), which might account for a possibility of recall bias. Second, in this cross-sectional study, the causal relationship between daytime napping and successful aging could not be established. Third, our study contains no qualitative data on daytime napping. Thus, one cannot disentangle the underlying mechanisms of the association between napping and successful aging. Fourth, some potential confounders (e.g. depression, medication use) of the relationship between daytime napping and successful aging were not incorporated in our study due to unavailability of data. Last, the study sample was conducted in China, where cultural norms and patterns of daytime napping may be different from that in other countries, thereby limiting the generalizability of our findings to other groups globally. Nevertheless, to our knowledge, this is the first study to examine the association between daytime napping and successful aging using a large nationally representative sample.
Future studies may benefit from using longitudinal data to explore the causal relationship and using objective measurements of sleep such as actigraphy and polysomnography [
45] to reduce recall bias. Also, Mendelian randomization studies are suggested to be conducted to avoid residual and unmeasured confounding effects. Furthermore, biological and social factors affecting the relationship between daytime napping and successful aging need in-depth exploration in future research.
Applications
Based on our study findings, interventions promoting successful aging could target older adults with long duration of daytime napping, especially those who sleep more than 8 h/night. Increased exercise and leisure activity opportunities could be provided for the elderly [
46,
47] and psychosocial care could be encouraged to help older adults to attain successful aging [
48].
For the older adults with cognitive impairment, moderate daytime napping could be recommended, especially among those whose nighttime sleep is inadequate. In addition, population with major diseases and risk factors for diseases (i.e. diabetes; cancer; chronic lung diseases, heart attack, stroke, emotional or psychiatric problems, asthma, hypertension, and obesity) may be recommended to avoid daytime napping, especially those who sleep 6 h and above per night. To improve the chances of successfully modifying and maintaining napping behavior, these recommendations could be provided by physicians in hospital settings [
49] when older adults are diagnosed or followed up with health issues.
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