Are you one of those people who rises before dawn and never needs an alarm clock? Or would you happily sleep until midmorning if you could? Do you feel like you are just hitting your day's stride by late afternoon, or do you like to get the big tasks of the day accomplished early?
Most of us have some degree of preference for late nights or early mornings. Where an individual falls on this spectrum largely determines his or her chronotype -- an individual disposition toward the timing of daily periods of activity and rest. Some of us are clearly "larks" -- early risers -- while others of us are distinctly night owls. The rest of us fall somewhere in between the two.
We're learning that these night owl and early riser tendencies are driven by some significant degree by biological and genetic forces. Different chronotypes are associated with genetic variations, as well as differences in lifestyle and mood disposition, cognitive function and risks for health problems, including sleep disorders and depression.
New research has now found evidence of physical differences in the brains of different chronotypes. Scientists at Germany's Aachen University conducted brain scans of early risers, night owls, and "intermediate" chronotypes who fell in between the two ends of the spectrum. They discovered structural differences in the brains of people with different sleep-wake tendencies. Researchers observed a group of 59 men and women of different chronotypes: 16 were early risers, 20 were intermediate sleepers, and 23 were night owls. They found that compared to early risers and intermediates, night owls showed reduced integrity of white matter in several areas of the brain. White matter is fatty tissue in the brain that facilitates communication among nerve cells. Diminished integrity of the brain's white matter has been linked to depression and to disruptions of normal cognitive function.
The cause of this difference in quality of white matter among night owls compared to other sleepers is not clear. Researchers speculate that the diminished integrity of white matter may be a result of the chronic "social jet lag" that characterizes the effects of the sleep-wake routines of many night owls. People who are disposed toward staying up late and sleeping late often find themselves at constant odds with the schedule of life that surrounds them, particularly work and school schedules that require early-morning starts. This can leave night owls chronically sleep deprived, and experiencing many of the same symptoms -- fatigue and daytime sleeplessness, difficulty focusing, physical pain and discomfort -- of travel-induced jet lag.
Research indicates that people who stay up late are at higher risk for depression. Studies have also shown night owls more prone to more significant tobacco and alcohol use, as well as inclined to eating more, and also less healthful diets than early risers or people with intermediate sleep patterns. But research on the influence of chronotype isn't all bad news for night owls. Some studies have shown that people who stay up late are more productive than early risers, and have more stamina throughout the length of their days. Other research has shown that night owls display greater reasoning and analytical abilities than their earlier-to-bed counterparts. Stay-up-late types, according to research, achieve greater financial and professional success on average than those people with earlier bedtimes and wake times.
This latest study is the first to offer physical evidence of neurological differences among people with different sleep tendencies. But other research has also shown that the inclinations toward staying up late or rising early are deeply rooted in biological and genetic differences:
Scientists have discovered an "alarm clock" gene that activates the body's biological clock in the morning from its period of overnight rest. Identifying this gene and its function may eventually tell us important new information about the influence of chronotype and circadian function on sleep and health.
Research has also revealed differences in brain metabolic function among night owls compared to early risers and middle-of-the-road sleepers. These metabolic differences were discovered in regions of the brain involved in mood, and may be one reason why night owls are at higher risk for depression related to insomnia.
Recently, scientists identified a gene variant that exerts a strong influence over the circadian clock, and with the inclination to stay up late or rise early. This genetic variation -- which affects nearly the entire population -- can shift the timing of an individual's 24-hour sleep-wake cycle by as much as 60 minutes.
If our preferences for sleep and wake times are strongly influenced by genetics and biology, what are we to do when faced with inclinations that don't match up with the demands and responsibilities of our lives? Genetic forces appear to play an important role in our preferences, but we're still working to understand just how, and how much. And we're far from powerless: The choices we make about our sleep environments and sleep habits can also make a significant difference. A recent study showed that limiting nighttime exposure to artificial light and increasing exposure to daytime sunlight can shift sleep-wake cycles earlier -- even for night owls. Strong sleep habits -- being careful about alcohol consumption close to bedtime, sticking to regular sleep and wake times, making sure your bedroom is dark and electronic-gadget free -- can help reinforce your sleep schedule, even if it doesn't align perfectly with your natural tendencies.
More broadly, I hope we'll see society begin to recognize the power of these biological sleep patterns, and the need for flexibility to enable people to construct work and school schedules that align better with their dispositions toward sleep. This is a smart, sleep-friendly strategy that would be good for public health and productivity.