Integrating nap and night-time sleep into sleep patterns reveals differential links to health-relevant outcomes

Both night-time sleep and nap behaviour have been linked consistently to health outcomes. Although reasons for napping are usually tied to night-time sleep, the majority of studies assess their effects independently. The current study thus aimed to examine the health relevance of patterns of sleep behaviour that take into account both night-time and daytime sleep habits. Night-time sleep, recorded during 7 days via actigraphy from 313 participants (aged 34-82 years) of the Midlife in the United States II Biomarker study, was assessed. Blood and urine specimens were assayed for noradrenaline, interleukin-6 and C-reactive protein. Participants self-reported nap behaviour, depressive symptoms, perceived chronic stress and the presence of medical symptoms and conditions. Overall, nappers (n = 208) showed elevated waist-hip ratios, C-reactive protein and interleukin-6 levels compared to non-nappers and reported more physiological symptoms and conditions (all P ≤ 0.019). Within nappers, cluster analysis revealed three patterns of sleep behaviour-infrequent nappers with good night-time sleep, frequent nappers with good night-time sleep and nappers with poor night-time sleep. Nappers with poor night-time sleep thereby exhibited elevated noradrenaline levels, depressive symptoms and perceived stress scores compared to other groups (all P ≤ 0.041). These findings support the idea that nap-health relationships are complex, in that frequency of napping and accumulation of nap sleep is not related linearly to health consequences. Assessing nap behaviour in conjunction with night-time sleep behaviour appeared crucial to elucidate further the health relevance of napping, particularly in terms of psychological health outcomes, including chronic stress and depressive symptoms.