Sleep Timing
The optimal timing to go to sleep is determined by our biological clock which runs a little early for some people and a little late for others.
In Western countries, the most common time for adults to start to sleep is between 11pm-midnight. However, sleep schedules vary a lot from one person to the other. The time people like to wake-up and sleep is known as chronotype.
Chronotypes
People who go to bed and wake-up early are known as morning chronotypes (Morning Larks), while those who stay up later and like to sleep in later are referred to as evening chronotypes (Night Owls). These individual differences are perfectly normal and healthy in most cases. Our chronotype reflects the internal time of our biological clock. The main biological clock is located in our brain. It regulates daily patterns of physiology and behaviour. As such, it influences when we want to fall asleep and wake up, and controls daily rhythms in such things as hormone release, metabolism, cardiac function, eating and digestion. Health and wellbeing are maximized when these occur at the right times of the day for your body.
Your chronotype, and thus your optimal sleep timing, is determined by several factors:
Your age has a big influence on chronotype. Teens and young adults often have a later chronotype. Children and the elderly tend to be more early types.
Sex also can influence chronotype, as males tend to have later chronotype than females.
Genetics play a big role in chronotypes as well, with some people just naturally being earlier or later types relative to others.
Lifestyle features can alter our chronotype. Spending more time indoors and exposed to artificial light rather than natural sunlight tends to give people a later chronotype.
The social clock that dictates our work and school schedules also influences our chronotype.
While variation of chronotypes between people is perfectly natural and normal, this can become a problem for extreme morning larks and extreme night owls. Adults who prefer to go to sleep before 8:30pm and awaken spontaneously before 5:30am may have Advanced Sleep-Wake Phase Disorder, a condition that affects about 0.25% of the populations. On the other end of the spectrum, adults who prefer to go to sleep after 3am and wake up after 11am may have Delayed Sleep-Wake Phase Disorder. Delayed Sleep-Wake Phase Disorder is much more common and can reach rates of up to 16% in young adults. In many cases these conditions can be related to age or lifestyle, but in other cases these have biological causes, particularly when this trait runs in a family. Genetic studies have found that heritable versions of these conditions are caused by variation in the genes that make up our circadian clock.
Our circadian clock determines the time that we naturally want to go to sleep and wake up. However, our circadian clock is often in conflict with social schedules. If we need to wake up at different time on a work/school day than we naturally do on a day off, we are experiencing what is known as Social Jet Lag. The bigger the time difference, the more this can impact our mental and physical wellbeing. Chronic social jet lag can lead to difficulties with attention, memory, and higher thinking skills like decision making. They may also have poorer cardio-metabolic health.
Recommendations to better manage sleep timing:
It can be hard to harmonize our natural chronotype with some social schedules. Here are a few hints that may help:
If we have flexibility in choosing our daytime schedule, understanding our natural chronotype can help us select the optimal timing of classes, positions with work shifts, and other daily activities that minimize social jet lag.
Exposure to bright light (ideally natural sunlight!) can help shift the biological clock earlier or later so that our biological rhythms are better scheduled to help us fall asleep and wake up at the desired time.
Evening light delays the biological clock later: Helps people with early chronotypes to go to sleep and wake up later if they need to.
Morning light shifts the biological clock earlier: Helps people with too late chronotypes to wake up earlier and fall asleep earlier if they need to.
Maintaining a regular sleep-wake cycle, even on days-off, and aiming to exercise and eat at regular schedules, can help anchor the timing of the biological clock. This, in turn, can help stabilise sleep timing, which we know maximizes health and wellbeing.