Irregular sleep timing tied to bad mood, depression

Having irregular sleep timing can increase the risk of depression, as much as short sleep duration or frequently staying up late at night, suggests a recent study.

While short sleep duration is a known driver of depression, researchers are just beginning to understand the association between regularity of sleep timing and mental health, thanks to a large study done in over 2,000 medical interns during their first-year residency training at the University of Michigan. 

Given that the first year of medical residency training is often characterized by increased workload and shifting schedules daily, this therefore provides a prospective model to study the association between sleep variability and mood disorder.

“Our findings aim not only to guide self-management on sleep habits but also to inform institutional scheduling structures,” said lead author Dr Fang Yu from Michigan Neuroscience Institute, University of Michigan in Ann Arbor, Michigan, US.

In the study, 2,115 first-year medical interns were followed on their sleep habits using a commercial wearable device (Fitbit) and assessed on daily mood using a smartphone application and depressive symptoms with the 9-item Patient Health Questionnaire (PHQ-9). [npj Digital Medicine 2021;doi:10.1038/s41746-021-00400-z]

Participants with the most irregular sleep schedule — as indicated by variable wake time (b=0.081; p=0.005) and 24-h total sleep time (TST; b=0.4; p=0.0012) — were more likely to be depressed than those with less variable timing.

Similarly, participants who slept fewer hours (ie, shorter TST; b=−0.11; p<0.001) and regularly stayed up late (b=0.068; p=0.015) also scored higher on depressive symptoms.

Even when it comes to just their mood the following day, participants with the most variable sleep schedule were just as likely to be in a foul mood as those who regularly slept later or had fewer hours of sleep daily.

Within these individuals, sticking to a more regular sleep schedule in terms of TST (b=−0.011; p<0.001) and wake time (b=−0.004; p<0.001) appeared to be a remedy for improved mood the following day; so were earlier bedtime (b =− 0.07; p<0.001), longer TST (b=0.06; p<0.001), and later wake time (b=0.09; p<0.001).

“These findings highlight sleep consistency as an underappreciated factor to target in depression and wellness,” said study co-author Dr Srijan Sen, also from Michigan Neuroscience Institute. “The work also underscores the potential of wearable devices in understanding important constructs relevant to health that we previously could not study at scale.”

“These findings have implications far beyond medical trainees … By identifying variability in sleep duration and timing as a potential factor associated with mood, this modifiable behaviour could be considered more broadly as part of a multifaceted approach to optimize mental health in general adult populations,” said Fang and co-authors.