No matter how long you’ve been awake, the intensity of brain activity during the day appears to increase your need for sleep, proved by a new UCL study in zebrafish.
Two systems regulating sleep
The two systems are called circadian and homeostatic systems.
Circadian system is a roughly 24-hour cycle, commonly referred to as a clock that controls alertness, sleep, hormone production, body temperature and organ function. When it comes to circadian system regulating sleep, it means your internal clock to sleep and wake up.
Homeostatic system is a set of body components, functionally connected, that effect regulatory compensatory responses to maintain a particular physical or chemical factor in the internal environment (extracellular fluid) relatively constant around a “set” of “operating” point.
Understanding of the systems in sleep
In previous studies, researchers have already figured out how the circadian system times our biological rhythms including sleep cycles. But the homeostatic system, which causes us feel increasingly tired after a very long day or sleepless night, is not well understood.
Researchers observed that it appears to be driven not only by the awake time, but the intensity of brain activity. To understand the processes in the brain, they studied zebrafish larvae.
Study in zebrafish
Zebrafish are commonly used in biomedical research for two reasons: the near-transparent bodies which facilitate imaging, and the similarities to humans such as sleeping every night.
In the study, zebrafish are stimulated to increase brain activity with caffeine and other substances. It turned out that zebrafish with their brain stimulated by drugs slept longer after the drugs had worn off.
The results confirmed that the increase in brain activity contributed to a greater need for sleep.
What this is all about
Researchers looked further into the mechanism, and found that one specific area of the zebrafish brain was central to the effect on sleep pressure. The area is comparable to a human brain area found in the hypothalamus which is active during sleep.
In the area, a brain signaling molecule called galanin was particularly active during recovery sleep, but did not play as big a role in regular overnight sleep.
A confirmation study
To confirm the findings mentioned above, the researchers conducted another test. In the test, they imitate fast-flowing water to make the young zebrafish think they need to keep swimming to avoid moving with the water stream.
The imitation lasted for a whole night, keeping the zebrafish awake all the time. The zebrafish slept more the next day, and their brains showed an increase in galanin activity during recovery sleep.
The researchers say their finding that excess brain activity can increase the need for sleep might explain why people often feel exhausted after a seizure.
The researchers are discovering the gene that plays a central role in homeostatic sleep regulation. The findings may help to understand sleep disorders and conditions that impair sleep, such as Alzheimer’s disease.
"We may have identified a good drug target for sleep disorders, as it may be possible to develop therapies that act on galanin," said Dr. Sabine Reichert from UCL Cell & Developmental Biology.