The researchers—led by Christina Schmidt (of the Cyclotron Research Centre and the Cognitive and Behavioral Neuroscience Centre, both at the University of LiÃ¨ge, Belgium; and Philippe Peigneux of the Cyclotron Research Centre and the Neuropsychology and Functional Neuroimaging Research Unit at the UniversitÃ© Libre de Bruxelles, Brussels, Belgium—looked at the circadian clock and the sleep homeostat.
The circadian clock regulates the body’s cycle of sleeping and waking by sensing light or darkness, specifically helping to control blood pressure and body temperature; for example, it lowers body temperature during sleeping hours.
The master circadian clock is hardwired in the brain, and basically tells your body when you like to get up in the morning--early, late, or in-between.
The sleep homeostat is a clock-like regulator that senses the amount of time a person has been awake or asleep; for example, when a person has been awake for a long time, it tells the body it is tired and it is time for sleep.
The sleep homeostat basically keeps track of how long you have been asleep or awake and tells you that after so many hours it is time to wake up or to go to sleep.
The article written by these researchers (“Homeostatic Sleep Pressure and Responses to Sustained Attention in the Suprachiasmatic Area”) appeared in the journal Science on April 24, 2009.
The researchers state in their paper, “Throughout the day, cognitive performance is under the combined influence of circadian processes and homeostatic sleep pressure. Some people perform best in the morning, whereas others are more alert in the evening. These chronotypes provide a unique way to study the effects of sleep-wake regulation on the cerebral mechanisms supporting cognition.”
They use the word “chronotypes” to describe groups of people with different sleep-and-wake cycles.
Page two continues.
The researchers used functional magnetic resonance imaging (fMRI) scans when performing the study, which analyzes blood flow and activity in various regions of the brain.
They found that 66% of people woke up later than the very early risers and before the very late risers—that is what is considered the more normal times to wake up in the mornings.
The researchers used volunteers who spent two nights in a laboratory setting. They were awake and asleep depending on their own unique circadian cycles.
During this time, two fMRI scans were performed on each volunteer, one two hours after waking up and a second one 10.5 hours after waking up. During the two scans the volunteers performed a test to determine mental alertness.
The researchers found that the very early risers and the very late risers were both equally alert in the mornings. As the day progressed the early risers had reduced activity in the brain regions that controlled mental attentiveness—those areas involving the circadian master clock, which regulates alertness and sleep-awake cycles.
They also performed mental tests less efficiently than late risers.
In addition, during the evenings, the early risers felt more tired and performed less efficiently on the mental tests than the late risers.
Page three concludes.
Specifically, ‘early birds’ are equally alert two hours after waking up as are those ‘night owls.” However, those people who are very early risers in the morning are less attentive about ten hours after waking up. And, they are much more tired and sleepy in the evenings than people who are very late risers in the morning.
The suprachiasmatic part of the brain contains the circadian clock and the locus coeruleus is the part of the brain that controls sleepiness and wakefulness.
The researchers found that the circadian clock of early risers were more apt to cause early risers to be sleepy in the evenings than the later risers. In addition, the early risers fell into a “slow-wave sleep” quicker than the late risers when they did fall asleep in the evenings.
This type of sleep occurs when the sleep homeostat tells the body it is tired and it is time to sleep. In other words, early risers have less ability to resist the temptation to sleep when they begin to feel tired in the evenings.
They concluded that in extreme cases (very early risers and very late risers) “… that maintaining attention in the evening was associated with higher activity in evening than morning chronotypes in a region of the locus coeruleus and in a suprachiasmatic area (SCA) including the circadian master clock. Activity in the SCA decreased with increasing homeostatic sleep pressure. This result shows the direct influence of the homeostatic and circadian interaction on the neural activity underpinning human behavior.” [Paper]
The researchers concluded that people are either early risers or late risers depending on whether the area of the brain the involves the circadian clock or the area of the brain that involves sleep homeostat is more active.
For people that are neither early risers nor late risers, what the researchers would call normal risers, these two areas are about equally active.
Dr. Peigneux stated, “Morning types may be at an advantage, because their schedule is fitting better with the usual work schedule of the society. It may represent a problem for evening types obliged to wake up early while having difficulties going to bed in the evening, eventually leading to a sleep debt." [National Geographic News: “Night Owls Stay Alert Longer Than Early Birds ”]
According to the Discover Magazine article Night Owls Have More Staying Power Than Early Birds, Brain Study Shows, “The study is the first to show that circadian rhythms and sleep pressure interact to govern behavior; researchers previously believed that the two systems operated independently.”
Although late risers appear to be more attentive during the day and less sleepy and tired in the evening, they do not alway have an easy time of fitting into society’s general routine of working from around 8 a.m. and 5 p.m. during the work week.
The researchers made the comment that late risers, who don’t get up until noon when on their own, are probably sleep deprived when they are forced to get up early to meet their career obligations.