This study, the first survey on the prevalence and factors associated with poor sleep among nursing staff in China, showed that the prevalence of poor sleep, as defined by a CPSQI score higher than 5, was 56.7 % [
15], remarkably higher than the 8–30 % in the general population in China [
3,
16]. By working rotating-shift schedules, individuals are exposed to work at the low point of their well-established circadian pattern, and their normal circadian systems are disrupted, which may ultimately result in poor sleep [
7]. Many previous studies have shown that night work and an unstable shift schedule can impair mental health and lead to poor sleep [
17‐
19]. Our finding showed that shift work was an independent risk factor for poor sleep, in agreement with previous epidemiologic studies. In Iran, of 925 health care workers, 43.1 % complained of sleep problems. [
20] In addition, Takayuki reported that the insomnia prevalence among nursing staff was three to four times higher than that in the general population in Japan [
8]. This is the first assessment of the comorbidity rate of different types of primary headache and poor sleep among nursing staff. No similar data have been reported previously.
The present study showed that the comorbidity of poor sleep with migraine and CDH was markedly greater than that with TTH. Additionally, compared with nurses without headaches, the comorbidity rate of poor sleep and all types of primary headache was higher. The results are similar to those of a previous study showing that headache sufferers had a high rate of poor sleep and that the prevalence varied for different headache types [
21]. No large-scale epidemiological survey was available to describe sleep disorders and headache comorbidity rates in detail, but several small clinical studies have reached similar conclusions [
22]. The prevalence of sleep disorders was 85.9 % among migraineurs, significantly higher than that in a group without headaches in the US state of Mississippi [
23]. Also, Sancisi’s study suggested that CDH sufferers were 2.71 times more likely to have poor sleep than were episodic headache sufferers [
24], but no distinction could be made for specific types of episodic headache. Our survey made up for this shortcoming and showed that the rate of comorbid poor sleep among CDH sufferers was higher than that among episodic TTH sufferers, but not higher than that in episodic migraineurs. In agreement with previous studies, our findings showed that poor sleep quality could increase the prevalence of primary headache. A survey including 310 community-dwelling Hong Kong Chinese women aged 40–60 years suggested that women with insomnia disorder had a 2.2-fold increased risk of reporting recurrent headache, a 3.2-fold increased risk of migraine, and a 2.3-fold increased risk of TTH, after adjusting for anxiety and depression [
25]. In the present study, nurses with poor sleep were 2.229 fold as likely to suffer primary headache, 2.734 fold as likely to suffer migraine, 1.75 fold as likely to suffer TTH, and 6.467 fold as likely to suffer CDH, very similar to the results of the survey in Hong Kong. We also attempted to find out the effect of sleep quality on headache frequency and headache intensity. Previous research showed that headache of greater frequency was more strongly associated with insomnia than headache of lower frequency [
25]. In our study, poor sleepers tended to experience headache more frequently, but the difference was not significant. The reason for this difference may be relatively small samples and different frequency grouping. Few studies have directly investigated the relationship between sleep disturbance and headache intensity, but previous studies have shown that poor sleep may reduce the pain threshold [
26], leading to a more severe subjective feeling of headache. Our results supported this viewpoint. Our study had several strengths. First, it is the first study to assess the comorbidity of primary headaches and poor sleep in nursing staff. Second, it is the first study to confirm the bidirectional relationship between primary headache and poor sleep through epidemiological numerical values. Furthermore, the diagnosis of primary headaches met the latest ICDH-3-beta guidelines and was confirmed by a headache specialist.
Inevitably, there are several limitations in our study. Because of the limited conditions, we did not distinguish among specific types of sleep disorders. Also, we evaluated the prevalence and characteristics of primary headache over 1-year period, but assessed the sleep quality over a 1-month time period due to nurses’ limited memory for specific sleep conditions. In addition, previous studies suggested that shift-work-related sleep problems may be associated with several types of shifts (day shift, evening shift, and night shift) as well as with work schedules (different shift rotations) [
5], but we did not distinguish among the specific shift types in our questionnaire