“Early to bed and early to rise” might make you proverbially healthy, wealthy and wise, but it also means that you become more sleepy and less alert as your day goes on, according to a functional magnetic resonance imaging study by European researchers.
“We linked this reduced alertness to reduced activity in the suprachiasmatic nucleus area and the locus coeruleus,” says Christina Schmidt, a sleep researcher at the University of Liege in Belgium and lead author of the study. The suprachiasmatic nucleus and locus coeruleus are clumps of neurons known to promote alertness and regulate the body’s circadian sleep/wake cycle. The study also linked reduced activity in these areas to a greater buildup of what researchers call “homeostatic sleep pressure,” the physiological urge for slumber.
“These data indicate that morning types are less able to cope with increasing time awake at the end of a normal waking day,” says Schmidt.
“This is a major step toward a better understanding of the mechanisms underlying sleep/wake regulation in humans,” says Marie Dumont, a neuroscientist and sleep researcher at the University of Montreal whose lab is also active in the field but was did not take part in this research.
Scientists who study the sleep-wake cycle have long recognized that some people are “early birds” who perform better on cognitive tasks in the morning, while late-to-bed “night owls” perform better in the evening. But the difference between these two chronotypes is more complex than it might seem.
Early birds and night owls don’t merely go through their lives out of phase with each other, in terms of the normal circadian rhythm. The two groups also differ in their susceptibility to homeostatic sleep pressure. This urge to go to sleep builds up automatically during the day and can be measured by the relative amount of deep sleep one gets just after falling into slumber at night. It is considered one of the main regulators of sleeping and waking, yet its source has long been something of a mystery.
“The anatomical location of the sleep homeostat is still not known,” says Schmidt. “It has been shown to be functionally and anatomically independent of the suprachiasmatic nuclei and most probably represents a diffuse system.” Some researchers believe that it reflects the buildup of the neurotransmitter adenosine, which binds to certain classes of brain cells and inhibits their activity. (Caffeine blocks this binding.)
French researchers reported in 2003 that for early-riser human subjects, sleep pressure increased more quickly in the later parts of their daily routine, compared with late-risers. Canadian researchers reported in 2006 that in early-risers, a measure of sleep pressure also seemed to dissipate more quickly during sleep.
In the current study, published in the April 24 issue of Science, Schmidt and her colleagues in the laboratory of Philippe Peigneux studied 15 early birds and 16 night owls, young people matched for age and other variables but chronotypically extreme—on average the early birds went to bed and got up about four hours before the night owls.
Each subject was studied in a sleep lab for two consecutive nights, starting seven hours before going to bed at the subject’s accustomed time. The researchers measured electroencephalography (EEG) recordings during sleep, sampling of melatonin levels in saliva, and functional magnetic resonance imaging (fMRI) scans 1.5 and 10.5 hours after waking.
While being scanned in the fMRI, the subjects performed a standard set of reaction-time tasks meant to indicate their ability to sustain attention. The night owls were significantly faster on the evening test, and also reported feeling significantly less sleepy on average, compared with the early risers, even though both groups seemed to be at the same point of their subjective day”
The fMRI scans during these tasks suggested that the suprachiasmatic nucleus (SCN) and locus coeruleus (LC) regions were both significantly more active in the night owl group. The EEG recordings indicated that early slow-wave sleep, a measure of sleep pressure, was greater in the early birds, thus indicating an inverse relationship between sleep pressure on the one hand and SCN activity and a measure of alertness on the other.
Although the precise causal relationship isn’t clear, Schmidt and her colleagues speculate that the mechanisms of sleep pressure may suppress the SCN/LC based circadian clock signals that help keep a person awake and alert.
In any case, she says, the data now suggest that sleep pressure changes more rapidly in early risers, accumulating more quickly during the day and leaving them sleepier before bed—but then dissipating more quickly at night to leave them fresher in the morning.