Sleep is not a single state that stays the same all night. While you sleep you move in cycles between two main phases: non-REM sleep and REM sleep. The US National Heart, Lung, and Blood Institute (NHLBI) says the cycle starts over every 80 to 100 minutes, and there are usually four to six cycles per night.[1] You may wake briefly between cycles.
The Short Answer
- Light sleep (stages 1 and 2): Stage 1 is the transition from wakefulness to sleep. By stage 2 you are asleep.
- Deep sleep (stage 3): Also called slow-wave sleep. There is more of it early in the night.
- REM sleep: Your eyes move quickly, brain activity resembles wakefulness and dreaming usually happens here. There is more of it later in the night.
These points come from the NHLBI page on sleep stages.[1]
Light Sleep: Stage 1 And Stage 2
Non-REM sleep has three stages, defined by brain activity measured in sleep studies. Stage 1 is the transition between wakefulness and sleep, when you can wake easily. When you reach stage 2, you are asleep.[1]
You pass through these stages in every cycle, so you return to light sleep again and again through the night. AuraSleep shows this time as “Light” in the morning report.
Deep Sleep: Stage 3
Stage 3 is called deep sleep or slow-wave sleep, after a particular pattern seen in measurements of brain activity. You usually spend more time in this stage early in the night.[1]
The amount of deep sleep changes with age. According to the NHLBI, slow-wave sleep peaks in early childhood, drops sharply in the teenage years and keeps decreasing through adulthood; older people may have none at all.[1]
REM Sleep
REM stands for rapid eye movement. In this stage your eyes twitch and your brain is active; measured brain activity is similar to waking hours. Dreaming usually happens during REM sleep. Your muscles normally go limp, so you do not act out your dreams.[1]
There is more REM sleep later in the night. You get less REM sleep in cold temperatures, because your body does not regulate its temperature properly during this stage.[1] Newborns spend more of their sleep in REM.
How Much Should You Sleep?
The American Academy of Sleep Medicine (AASM) and the Sleep Research Society (SRS) recommend that adults sleep 7 or more hours per night on a regular basis to promote optimal health.[3] The NHLBI table by age lists 7 to 8 hours for adults 18 and older.[2]
How Are Sleep Stages Measured?
The clinical way to measure sleep stages is a sleep study called polysomnography. Electrodes are placed on the scalp, chin and the outer edge of the eyelids, with monitors on the chest for heart rate and breathing; the test measures how long it takes to fall asleep and to enter REM sleep.[4]
Phone-based sleep apps cannot take these measurements. AuraSleep estimates stages from your phone’s microphone and motion sensors and, with your permission, uses smartwatch data from Apple Health or Health Connect. That is why the report says “estimated”. AuraSleep is not a medical device and does not replace a laboratory test.
Reading Your Morning Report
- Do not fixate on a single night. One low score says less than your weekly trend.
- Remember that deep sleep clusters early and REM towards morning. On mornings when you wake early, your REM time may look short for this reason.
- Note things like room temperature, night noise and bedtime; comparing them on the trend screen makes patterns easier to see.
- If you keep waking up tired, stop breathing while snoring, or have long-lasting insomnia, see a doctor.