When you go to sleep affects your waking hours, too.

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Going to sleep before midnight matters—it can make a huge difference in our quality of sleep, productivity, and overall health. You may have heard the phrase "an hour of sleep before midnight is worth two after," and that's because we have an internal biological clock, or circadian rhythm, that sends signals to our brain that it's time for sleep once the sun goes down.

By going outside of our circadian rhythm, or disrupting it, we can potentially throw off our internal biological clock and therefore impact our physical and mental health. Here's how it works, and all the sleep benefits you can lose by making a habit of heading to bed post-midnight.

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How our bodies know what time it is

Our circadian rhythm helps our brains keep track of when we should be awake and when we should be asleep. It regulates our sleep while also signaling when to be alert and when to become sleepy based on light changes in our environment, like sunrises and sunsets. This ingrained behavior has been present for thousands and thousands of years and is shaped by earth's rotation around its axis. Without circadian rhythm, we aren't able to optimize how much energy our bodies expend and when, meaning it helps maintain all aspects of our lives.

"The circadian rhythm uses sunlight that enters our eyes to keep us on a 24-hour schedule," explains sleep specialist and neuroscientist Chelsie Rohrscheib, PhD. "Light entering our eyes makes the brain suppress melatonin, one of the main hormones that regulates sleep."

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Melatonin is a hormone that sends a signal to your body to sleep. Rohrscheib says that as it gets darker in the evening, and less light information enters our eyes, our melatonin production increases until it reaches a certain threshold, at which point it signals to the brain that it's time for sleep. 

Generally, this melatonin signal occurs well before midnight. The longer we stay awake after this point, the more we shake up our natural sleep cycle. "When we cross time zones, this process is disrupted," describes sleep expert and researcher Nerina Ramlakhan, PhD, physiologist, sleep expert, and author of The Little Book of Sleep: The Art of Natural Sleep. "Environmental cues such as temperature and light levels interact with the circadian timer telling the body what time it is."

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Why sleeping before midnight is good for our health

In addition to regulating circadian rhythm, sleep before midnight can affect our overall wellness when awake. "Sleeping before midnight helps to ensure that you have enough daytime hours of light exposure to regulate your melatonin production," Rohrscheib says. "Not waking up early enough in the morning and not getting the sun exposure that you require to maintain your circadian rhythm is associated with difficulty falling asleep, staying asleep, and can impact your health."

Some studies have found that people who go to bed late and have trouble waking up in the morning are more likely to have a shortened lifespan, in addition to a much higher risk of psychological disorders and diabetes. "People who don't have a well-regulated circadian rhythm are more likely to suffer from cognitive deficits like poor focus, inhibited learning, and reduced memory consolidation," Rohrscheib explains. "People with an abnormal circadian rhythm are also at an increased risk for certain mental health conditions, like depression."

Ramlakhan explains that sleeping before midnight is important for setting the rhythm of sleep throughout the night, too, and can enrich our physical and mental health. "The pre-midnight sleep seems to bring about the most powerful repair to the brain and body," she says. "It sets the body into a good rhythm thereafter. In my experience, people who go to bed before midnight tend to have better daytime routines and are more likely to eat healthy and exercise regularly."

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What happens when we go to bed after midnight?

Going to bed after midnight on occasion is OK—and often unavoidable (you do have to live your life!). It may not always be possible to get to sleep early, or you may be up later for an event, or dealing with sick or crying kiddos. However, going to bed after midnight regularly or all the time can lead to a variety of health problems down the line.

Going to bed after midnight "can lead to chronic exhaustion and fatigue and even thyroid problems and burnout," Ramlakhan says. According to Ramlakhan's research and experience working in the field of sleep science, many people who go to bed later tend to oversleep the next day. This can lead to waking up with sleep inertia, or waking up in the transitional state between sleep and wake, earmarked by impaired performance, reduced vigilance, and a desire to return to sleep.

"This particular poor sleep pattern can affect your mood, as well as your physical health," Ramlakhan adds. Therefore, going to sleep before midnight is a healthy goal to strive for. While it may not always be feasible, experts recommend getting those crucial hours of sleep before the clock hits 12 a.m. as much as possible. For concerns about falling or staying asleep, it's important to speak to your doctor to develop a treatment plan or discuss how to modify your sleep habits.

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