No one feels good if they don’t get enough sleep. And evidence suggests that consistency in people’s sleep patterns — having a regular bedtime is also part of getting a healthy night’s sleep.

Doctors-in-training tend to lose out in both of these areas. Not only is their sleep likely to be interrupted when they are on call overnight, their schedules are likely to vary widely from day to day and week to week, making a consistent bedtime nearly impossible.

Variation in bedtime did not seem to increase the risk of depression or bad mood, but wake-up time seemed to be an important factor.

Researchers looked at the relationship between sleep, mood and depression risk in more than 2,000 first-year medical interns as part of the Intern Health Study. The long hours and variable work schedules interns and resident faced during that part of medical training resulted in sleep changes that had negative effects on their mental health.

Interns’ sleep was tracked using a Fitbit wrist sensor, their mood was assessed daily using a smartphone application, and their risk of depression was assessed using a quarterly questionnaire. “Advanced wearable technology allows us to study the behavioral and physiological factors of mental health, including sleep, at a much larger scale and more accurately than before,” Yu Fang, lead author on the paper, told TheDoctor.

Before the participants started their first year of residency, about two weeks’ worth of data were collected. Four more months of data were collected throughout the year.

Young doctors who had less sleep and a later bedtime, as well as greater variability in total sleep time and wake-up time for prolonged periods, had an increased risk of depression. Less sleep and a later bedtime also had a negative effect on their moods the next day, as did an earlier wake-up time and a large shift in total sleep time.

The difference between the effect of wake-up time and bedtime was surprising, said Fang. Variation in bedtime did not seem to increase the risk of depression or bad mood, but wake-up time seemed to be an important factor. “Bedtime is something you have more control over. Wake-up time is more closely linked to external stressors,” Fang explained.

The study findings can help people improve their sleep habits, she explained. People can adjust their bedtime based on how much sleep they need. But they have less control over what time they must wake up, since wake-up time is determined by external demands, which can impact depression risk and mood. Fang, a research specialist at Michigan Neuroscience Institute, believes people who can control their bedtime should try to keep it as consistent as possible.

The Intern Health Study enrolls a new group of interns every year. The researchers also want to look at different stages of sleep, such as rapid eye movement (REM) sleep or deep sleep, to see if changes in them are associated with mood.

The study is published in npj Digital Medicine.