Updated: Jun 28, 2020
By Claire Yuan, Copy Editor
When people turn in for the night, they tend not to think about what goes on inside their bodies and minds during sleep. Isn’t sleep just when your body kind of turns off? Well, no. During sleep, your brain is actually very active.
The first stage of sleep is NREM-1. During this stage, sleep is light and the sleeper can easily awake. Muscle activity and eye movement slows, and the brain produces alpha and theta waves. It is during NREM-1 that sleepers experience the sensation of falling, typically followed by muscle contractions. This stage of sleep is relatively short, lasting up to seven minutes.
In NREM-2, a slightly deeper stage of sleep, the brain produces a sudden increase in brain waves known as sleep spindles. Then, brain waves slow down, body temperature drops, and heart rate slows as the body prepares for deep sleep in upcoming stages. Waking up during NREM-2 is ideal for a “power nap” during the day.
NREM-3 is the deep sleep stage. During this stage, the brain produces extremely slow delta waves interspersed with smaller, faster waves. In addition, eye movement and muscle activity stop, and parasomnias such as night terrors, sleepwalking, bedwetting, and sleeptalking may occur. The sleeper is extremely difficult to wake during this stage, as the body becomes less aware of outside stimuli, and, upon being awakened, may be disoriented for several minutes.
Similar to NREM-3, during NREM-4, the brain exclusively produces delta waves and the body moves into an even deeper, more restorative stage of sleep. During the deeper stages of sleep, the body repairs tissues, stimulates growth, fortifies the immune system, strengthens bone and muscle, and builds up energy for the next day.
The final stage of sleep, REM, is generally entered around an hour and a half after initially falling asleep. As its name, Rapid Eye Movement, implies, this stage is characterized by bursts of rapid eye movement. Dreaming also happens mostly in this stage, and it is theorized that the bursts of eye movement are associated with visual dreams. During REM, most muscles of the body (except important muscles such as the heart and diaphragm) are paralyzed. Heart rate and blood pressure increase during REM sleep, and breathing often becomes fast, shallow, and irregular.
For a typical adult, these stages cycle through five or six times over a night of sleep. However, these stages last for varying lengths of time at different ages. As people age, they tend to sleep lighter and get less deep sleep. For example, a baby may spend up to 50% of their sleep in the REM stage, compared to 20% for most adults. As a person moves through the cycles of sleep, the REM stage increases in length, which is why it is likely that we wake from dreams in the morning.