The sleeping brain: changing rhythms and active memory work
The sleeping brain continues working throughout the night. Measurements of its activity distinguish several stages, and during rapid eye movement sleep its activity resembles that recorded during waking. Sleep moves through REM and non-REM phases in cycles that usually restart every 80 to 100 minutes. A night commonly contains four to six such cycles, with short awakenings possible between them. Sleep researchers use sensors to track brain activity and eye movements, allowing them to identify the phases through recorded signals. The sleeping brain produces changing patterns throughout the night rather than a single inactive state.
Non-REM sleep has three stages. The first marks the transition from wakefulness, the second is established sleep, and the third is deep or slow-wave sleep, named for its measured brain activity. Deep sleep is generally more prominent early in the night, while REM occupies more time later. During REM, the eyes move and muscles usually become limp, limiting the acting out of dreams. Sleep patterns also change with age: newborns spend more time in REM, while slow-wave sleep peaks in early childhood and declines sharply in the teenage years, continuing downward through adulthood.
Timing depends partly on internal clocks that generally follow a roughly 24-hour rhythm. A central clock in the brain coordinates sleep timing, while other clocks operate in organs throughout the body. Environmental cues, particularly light and darkness, help synchronize these rhythms. Signals arriving through the eyes reach the suprachiasmatic nucleus, which helps convey timing information through the nervous system. Circadian rhythms affect cells, tissues, and organs across the body. Many people's natural cycle is slightly longer than 24 hours, and individual timing differs, including the later schedules common among teenagers.
Another influence is sleep pressure, the biological need that rises with extended wakefulness. Adenosine accumulates in the brain while a person is awake and is linked to that growing pressure; caffeine can block its action. The light-dark cycle also influences melatonin, a hormone released into the blood whose levels rise in the evening and peak in the early morning. Melatonin is thought to promote sleep. Increasing light accompanies release of cortisol, which prepares the body to wake. Bright artificial light late in the evening can interfere with the usual melatonin signal and delay sleep.
Changes in the sleeping brain and nervous system extend to circulation. During non-REM sleep, blood pressure and heart rate fall as parasympathetic activity predominates. During REM and waking, sympathetic activity increases, bringing those measures toward the levels of relaxed wakefulness. Breathing becomes less frequent and less deep during sleep, with lower oxygen intake. Hormone production follows daily patterns as well, and the pulses of certain reproductive hormones grow larger near puberty. These shifts accompany sleep across multiple systems, with different stages and times bringing different patterns rather than the same physiological conditions for an entire night.
Sleep also supports learning and the formation of long-term memories. Insufficient or poor-quality sleep can impair concentration and clear thinking, while what happens overnight affects how someone feels when awake. Immune activity varies across the daily cycle, and a particular type of immune cell works harder during sleep. Metabolism has its own circadian organization: clocks in the liver, fat, and muscle help coordinate the handling of nutrients. The liver's preparation to digest fats is one example. Sleeping bodies continue the work of maintaining physical health while supporting functions that matter during the following waking period.
A study published in Science in September 2019 examined a possible role for sleep in forgetting. Thomas Kilduff of SRI International worked with teams in Japan to investigate neurons in mice that produce melanin concentrating hormone, or MCH. These cells lie in the hypothalamus and send connections toward the hippocampus, a brain region important for memory. They were already known to participate in sleep and appetite. Researchers used staining to identify the connections, then genetic tools to activate or silence the neurons during tasks that tested the animals' memory for objects they had encountered.
When those neurons were activated during the period between learning and long-term memory storage, the mice remembered less well. Turning the cells off or removing them improved performance. The effect depended on sleep state: suppressing their activity during REM improved memory, while doing so during waking or other sleep states did not. The results suggested that these neurons help the mouse brain actively forget new information. Researchers proposed that such activity could also help explain why dreams are quickly forgotten. In these measurements of the sleeping brain and waking brain, 53% of the neurons fired during REM, about 35% during waking, and 12% in both states.
- NHLBI, How Sleep Works: Sleep Phases and Stages — www.nhlbi.nih.gov
- NHLBI, How Sleep Works: Your Sleep/Wake Cycle — www.nhlbi.nih.gov
- NHLBI, How Sleep Works: Why Is Sleep Important? — www.nhlbi.nih.gov
- NIH Research Matters, REM sleep may help the brain forget — www.nih.gov