It’s a familiar question from your health-care provider: Are you getting enough sleep? Studies have shown that adequate sleep, between seven and nine hours for adults, can improve cognition, mood and immune functioning. But new research reveals that it’s not just hours of sleep that count toward mental health benefits. It’s whether that sleep occurs on a regular or irregular schedule.

An NPJ Digital Medicine study published in February looked at the sleep habits of more than 2,000 first-year medical residents. The researchers found that variability in sleep habits significantly affected their mood and depression — no matter how many total hours they slept.

Yu Fang, the lead study author and researcher at the University of Michigan’s Neuroscience Institute, summarized the findings in an email: “Keeping a regular sleep schedule is as important as, if not more important than, having enough sleep time for one’s mental health.”

Though few of us work the same long, unpredictable hours as medical interns, the study’s findings are still relevant, the experts said, particularly during the coronavirus pandemic, which hasn’t been kind to our mental health or to our sleep schedules. According to the Centers for Disease Control and Prevention, the prevalence of depressive disorder in June last year was 24.3 percent, roughly four times higher than the prior year. Meanwhile, many at-home employees are working into the evenings, and many Americans are suffering from “coronasomnia” due to chronic stress.

Suzie Bertisch, associate physician and clinical director of behavioral sleep medicine at Brigham and Women’s Hospital, calls the Michigan study a “springboard” for discussing the emerging science supporting the importance of a consistent sleep schedule. Other recent research, including  a 2018 Lancet Psychiatry study, and  2017 and 2015 studies in the journal Sleep, has found that disrupting circadian rhythms is related to mental health issues and that depression has a stronger association with interrupted sleep than with sleep deprivation

Importance of good sleep

According to Seth Davis, a Denver-based adult sleep coach, sleep is vital to our physical and mental well-being. While we’re in dreamland, our brains are processing the new information we’ve learned that day, activating our immune systems and producing growth hormones to help our bodies repair and recover.

“If you’re getting a good night’s sleep, you should be able to fall asleep within about 20 minutes, and pretty much sleep through the night,” said Kelly Murray, a Chicago-based adult sleep coach. You should also feel refreshed when you rise and maintain steady energy throughout the day.

When your sleep suffers, your mood does, too, Davis said. Situations that would normally be annoying can feel catastrophic as your patience evaporates more quickly than usual.  “You might feel more moody and emotional,” he said.

Fatigue affects our mood because of our biology, according to Murray. “It’s really unnatural for humans to not get enough sleep.” She said when we’re sleep-deprived, our body reacts as if we’re in danger and activates the fight-or-flight system, which explains why we tend to overreact. This is consistent with research showing a relationship between elevated cortisol, a stress hormone, and sleep loss.

Circadian rhythm and sleep

Although quantity of sleep matters, Bertisch said, “the timing of sleep is critical.” And the timing of sleep hinges on our circadian rhythm. The 24-hour circadian cycle determines the timing of hormonal fluctuations and variations in alertness and body temperature that prepare your body for wakefulness or sleep.

When you wake up, light exposure triggers the release of cortisol, “the alert hormone,” Murray said. As the day goes on, your cortisol levels decrease. Meanwhile, your body releases melatonin (the sleep hormone), about 12 to 14 hours after that initial cortisol surge.

Because of circadian rhythms, “you can’t sleep on demand,” Bertisch says. So if you need eight hours of sleep and you normally go to sleep at 11 p.m., you’ll rise around 7 a.m. But say one day you need to get up at 5 a.m. to make an early meeting. You probably won’t be able to fall asleep two hours earlier than usual the night before, because your body won’t be ready to sleep; therefore, you won’t get your usual eight hours.

Just like we generally can’t fall asleep on demand, we struggle to wake before the circadian rhythm has prepared our body to do so. According to physician Brian Gotkin, a pulmonary and sleep specialist at Memorial Regional Hospital in Hollywood, Fla., rising body temperature and cortisol levels signal your body to wake up; this process can be helped by light exposure. Then “the longer you’re awake, the more sleepy you get.”

Interrupting the cycle

If you stay up late to work or socialize and simply sleep longer the next morning to get your eight hours, you might think that’s okay. But any time you interrupt your natural schedule, Gotkin said, “you’re trying to do something your body doesn’t want to do.” People who sleep in over the weekend often feel out of sorts when they must get up earlier on Monday. Murray said this “social jet lag” occurs because your cortisol and melatonin levels are out of sync with your weekend schedule.

The specific mechanism by which sleep timing affects overall mental health is still not completely understood, said Fang, the researcher who studied the medical residents. But the link between inconsistent sleep schedules and mental health outcomes may have to do with sleep quality, said David T. Plante, a psychiatrist and medical director of the Wisconsin Institute for Sleep and Consciousness at the University of Wisconsin at Madison.  “If you change the timing of when you’re sleeping, you can really affect the quality of your sleep,” he said. Over time, “it can have a downstream effect on your overall well-being and mental health as well.”

How to maintain the cycle

To maintain a consistent sleep schedule, it’s important to stay as “in sync” as possible with your natural circadian rhythm. Here are tips.

Wake at the same time every day. According to Davis, rising at the same time every day (or as close to it as possible), including on weekends, is “one of the most powerful anchors to keep our circadian rhythm functioning regularly.” Bertisch suggests using an alarm to ensure a consistent wake time.

Eat and exercise at the same time. Try to eat and exercise at roughly the same times every day. Davis said this is particularly important if your schedule doesn’t permit you to get a sufficient amount of sleep. Keeping your other routines consistent offers “a cue for your circadian rhythm to stay on track.”

Also, avoid elevating your heart rate within three hours of your desired bedtime. According to Murray, a workout causes spikes in your cortisol levels and your body temperature, both of which should be decreasing in order to prepare your body for sleep.

Stick to a regular bedtime routine. Even if you can’t go to sleep at the same time every night, it’s important to prepare your body to wind down the same way every night. Murray suggests setting an alarm about 30 to 60 minutes before the time you plan to get into bed and avoiding light exposure and stimulation during this time. Dim the lights, unplug from work and avoid screens.

That said, if you have no problem falling asleep after watching a “Friends” rerun, there’s no reason to stop doing this, Davis said. He said the content itself, rather than the blue-light exposure, is the biggest threat to good sleep. In other words, don’t doom-scroll before bed.

Keep your naps short. “Naps can be kind of tricky,” Davis said. Longer naps in particular can disrupt your circadian rhythm by pushing your bedtime later than usual. If you really need a midday fade, he suggests sticking to “power naps” of up to 30 minutes.

Go outdoors. Murray suggests spending time outside (without sunglasses) as early in the day as possible. Even two minutes outdoors before 10 a.m. will help you perk up, she said; our eyes have neurons that “take data points from the sunlight” to signal our brains to release cortisol. Although some of us can get by with indoor light, Bertisch said, others need the brightness of natural light to improve their mood and alertness.

Going outdoors in the late afternoon can have the opposite effect, Murray said. The distinct quality of late-day light (including the angle at which it hits your eyes and the ratio of the different colors of light) tells your body that evening is approaching and boosts melatonin production.

“We are animals living on a spinning planet. One of the main drivers of when we sleep is what our circadian rhythm is and what our biological night is, and that is largely determined by sunlight and timing of light exposure,” Bertisch said. “The best way to sleep is actually having better routines.”

Pam Moore is a Boulder-based freelance writer, marathoner, Ironman triathlete and personal trainer. She’s also the host of the “Real Fit” podcast. Visit her at pam-moore.com.