Scientists once thought that sleep was a passive state, a time when a person's brain and body shut down for the night to rest and recover. But now, researchers know that sleep is a highly active time, a period during which the brain and some physiological processes may be hard at work.
For example, some hormones involved in growth in children, cell repair or digestion are boosted during sleep. Brain pathways involved in learning and memory also increase, according to the National Institutes of Health (NIH).
In fact, the brain is sometimes more active when a person is asleep than when he or she is awake, according to Harvard Medical School. But sleep can also slow down many other physiological processes, from heart rate and breathing to body temperature and blood pressure.
The stage of sleep a person is in also affects how active the brain and body are.
For more than 60 years, sleep researchers have known that there are two major categories of sleep: REM sleep, which stands for "rapid eye movement," and non-REM or non-rapid eye movement sleep, said Dr. Stuart Quan, clinical director of the division of sleep and circadian disorders at Brigham and Women's Hospital in Boston.
Non-REM sleep is now considered to consist of three stages, known as N1, N2 and N3, Quan said. Before 2007, non-REM sleep was broken down into four stages, but then sleep medicine specialists decided that there was no physiological reason to distinguish between two of the stages, the old stage 3 and stage 4 sleep, he explained. Those were combined into one stage, now referred to as N3.
During sleep, the brain repeatedly cycles through four distinct stages of REM and non-REM sleep in a specific sequence. This sequence changes somewhat between the first and second half of sleep. As sleep progresses in a series of four to five sleep cycles throughout the night, the time spent in the REM stage gets longer and the time spent in N3 sleep gets shorter, Quan said.
Live Science asked Quan for a more detailed explanation of what happens in the body and brain during each of these four stages of sleep.
When a person gets drowsy, he or she is drifting into N1 sleep, Quan said. In this first stage of non-REM sleep, a person is making the transition from being awake to falling asleep.
This is a relatively light form of sleep that lasts about 5 to 10 minutes. During this stage, heart and breathing rates begin to slow, eye movements also slow, and muscles relax. Body temperature decreases, and brain waves, if observed on an electroencephalogram (EEG) in a sleep lab, would be seen to slow.
A person can be easily awakened from N1 sleep, and that individual may not think he or she had been asleep, Quan said. N1 sleep is the first stage entered when taking a nap.
It's normal for a person to experience "hypnic jerks," also known as "sleep starts," during N1 sleep, Quan said. This is a sudden, brief muscle jerk that may happen along with a falling sensation when a person is in bed, he said. When it occurs, this sudden movement may or may not wake a sleeper up.
Adults spend the least amount of time in stage N1 sleep, which represents about 5 percent of their total sleep time, Quan said.
Shortly after N1 sleep ends, a person enters this second stage of non-REM sleep, which typically lasts 10 to 25 minutes, Quan told Live Science. It's also considered a period of light sleep.
During this stage, eye movement stops, heart rate slows, brain waves become slower and muscles relax even further.
As sleep cycles repeat throughout the night, a person spends more time in stage N2 sleep than in any other sleep stage, according to the National Institutes of Health. Adults spend about 55 percent of their total sleep time in stage N2 sleep, Quan said.
Non-REM sleep then progresses into its third stage, which is often referred to as "slow wave," "delta" or "deep" sleep. ("Delta" waves are a type of slow brain wave typically seen during this stage on EEG in a sleep lab.)
N3 sleep is a period of deep sleep that is needed for an individual to feel refreshed for the next day. A person typically spends more time in the N3 stage during the first half of sleep than the second half, but why this happens is not known.
Typically lasting 20 to 40 minutes, N3 sleep is when the brain becomes less responsive to external stimuli, and as a result, it is most difficult to wake a person up from this stage, Quan said. Someone awakened from N3 sleep is extremely groggy and disoriented, Quan said.
This grogginess is one reason why people may not want to nap for more than 30 minutes, because they can drop into N3 sleep, Quan said.
During N3 sleep, heart rate and breathing slow to their lowest levels during sleep. Blood pressure falls, and body temperatures drops even slower. Muscle activity decreases, and there is no eye movement. Blood pressure falls but not to a dangerous extent, Quan explained.
He said that this is also the stage when sleepwalking and sleep talking are most likely to occur. "Nightmares and night terrors are also N3 sleep phenomenon," Quan said. (Night terrors, also called sleep terrors, typically occur in children and involve a child sitting up in bed during sleep and screaming, according to the Mayo Clinic.)
Slow-wave sleep occurs for longer stretches in babies and young children, and the time spent in N3 sleep decreases steadily with age for reasons that are unclear, Quan said.
Adults typically spend about 15 percent of their total sleep time in stage N3, Quan said.
A person first enters REM sleep about 90 minutes after falling asleep and going through all three stages of non-REM sleep, Quan said. The first REM cycle of the night typically lasts about 10 minutes, but each subsequent REM stage gets progressively longer as the night goes on, he said.
The characteristic sign of REM sleep is that a person's eyes move rapidly from side to side beneath closed eyelids.
Although this eye movement is not constantly occurring, scientists don't know exactly why it takes place, although some have speculated that it's linked with dreaming.
Supporting that idea, REM sleep is the stage when most dreaming and vivid imagery occurs, Quan told Live Science. People often don't remember much of their dreams, but they are more likely to recall some aspects of a dream if awakened from REM sleep, he said.
During this kind of sleep, heart rate increases and blood pressure rises slightly compared with N1 sleep. Body temperature falls to its lowest point during sleep. Arm and leg muscles deeply relax to the point of being almost immobile, possibly to prevent people from acting out their dreams, according to the Mayo Clinic.
Breathing becomes fast and shallow, and the brain may be even more active during this stage of sleep than during wakefulness, sleep experts say. REM sleep is when the brain processes information from the day so it can be stored in long-term memory, according to the National Sleep Foundation, a nonprofit group that educates the public about sleep.
A newborn baby may spend about 80 percent of his or her total sleep time in REM-stage sleep, while infants spend at least 50 percent of their sleep time in REM sleep, according to the NIH. In comparison, most adults spend 20 to 25 percent of their total sleep time in REM sleep, Quan said.
The proportion of time spent in this state of sleep stays relatively constant throughout adulthood, but it may drop in people age 65 and older, Quan said. He added that sleep tends to be lighter in older adults, who experience more "microarousals," or blips into wakefulness. But these brief awakenings do not affect whether an older adult does or does not feel refreshed in the morning, Quan said.