We spend about a third of our lives asleep — which, for adults, is about eight hours of sleep for each 24-hour period (for babies through teens, the amount is even higher.) This sleep time is essential for virtually all aspects of our health and well-being, including appetite regulation, cardiovascular health, glymphatic processing, growth and healing, immunity, learning and memory, mental health, and weight control.
Nightly sleep cycles
There are four stages of sleep, and the amount of time we spend in each varies from person to person and from cycle to cycle. Generally, expect to go through approximately four to six cycles per night. The duration shifts as we continue to move through the non-REM stages (Stages 1 through 3) to rapid eye movement (REM) sleep and back again. Each sleep cycle lasts about 90 minutes, although this varies.
As for how all of this is determined: Sleep can be evaluated in 30-second increments using an EEG (electroencephalogram) and then “staged” based on the defining characteristics noted below.
Stage 1 sleep (transitional sleep)
This is the transitional phase in which we go from awake to asleep. It’s the lightest of the stages and the easiest to be awakened from. Hearing someone softly say your name — or any other slight disturbance — may be enough to rouse you. You may even twitch or have a sudden hypnic jerk (a sudden muscle contraction or even a feeling of falling) and startle yourself awake.
During this stage, you may even feel like you’re still awake. “But if we were to cue up the EEG (electroencephalogram), we would see sleep waves,” says Dr. Seema Khosla, medical director of the North Dakota Center for Sleep in Fargo.
Stage 1 is the shortest sleep stage, accounting for about 5% of total sleep time. However, when we’re experiencing very fragmented sleep (for example, waking multiple times for infant feedings), we may have more Stage 1 sleep as a result.
In general, Stage 1 sleep lasts for about one to five minutes. And it doesn’t only take place when you initially drift off: We enter this transitional stage multiple times, even though we may not be consciously aware of it.
Stage 2 sleep (light sleep)
Our Stage 2 sleep, known as light sleep, accounts for roughly half of our total sleep time. It’s important to note that it’s only light compared to our Stage 3 deep sleep; it isn’t “light” sleep the way our Stage 1 sleep is.
“Even though we call it light sleep, it doesn’t mean that it’s less valuable than deep sleep or REM sleep,” Khosla says.
As we cycle through the sleep stages, Stage 2 serves as the runway for the later stages, she explains. We may progress from Stage 2 into Stage 3 sleep but then return to Stage 2 sleep again before proceeding to the REM stage.
Physiologic changes during Stage 2 include a drop in body temperature, slower heart rate, and more regular breathing. This stage is also characterized by K complexes and sleep spindles.
The consolidation of memories is likely related to this spindle activity. It’s helpful to think of the stages as “collaborating and working together in a complementary manner,” she explains.
Stage 3 sleep (deep sleep)
This stage of sleep is known as deep sleep or slow-wave sleep. It’s characterized by slow waves, known as delta waves, as measured on an EEG. These build on the slow oscillations during Stage 2, Khosla notes.
Stage 3 is the deepest of the three non-REM stages and the hardest to wake from. If you are awakened during Stage 3 sleep, you’ll likely be groggy and disoriented. This is also the stage when behaviors such as sleepwalking, sleep talking, and bedwetting are most likely to occur in children.
During this stage, “the thinking parts of your brain are largely offline,” says Michael Grandner, Ph.D., director of the Sleep and Health Research Program at the University of Arizona. “Your brain is sort of in an automatic resting state... and certain functions are optimized to that state.”
As a result, “slow-wave sleep provides a neurophysiologic opportunity to perform maintenance, [including] synaptic pruning and glymphatic drainage,” he says.
In short, synaptic pruning is a consolidation of brain activity: Neural connections that aren’t being used are eliminated so that the remaining neurons can function more effectively. This process takes place throughout adolescence and into early adulthood.
There’s another “clearing out” function taking place during slow-wave sleep: flushing out “potentially neurotoxic waste products via the glymphatic system. (This system has been characterized as “the brain’s garbage disposal system.”)
Stage 3 sleep is the stage when most growth hormone is secreted — responsible not just for growth, as the name implies, but also for recovery and healing from injuries. The immune system is also strengthened during this stage.
Additionally, recent research indicates that the consolidation of memories that occurs in Stage 2 continues during the slow waves of Stage 3, with the two stages playing complementary roles. “Stage 3 is the downloading of short-term memory into long-term,” Khosla explains.
This stage, which typically accounts for roughly 20% of our sleep time as adults, is front-loaded so that more of our slow-wave sleep occurs during the first part of the night.
REM sleep
The final stage of sleep is REM sleep. (The previous stages of sleep are all categorized as non-REM sleep.)
Grandner describes REM as a state of “hot brain, cold body” in which our bodies become temporarily paralyzed so that emotional processing and other related functions can take place. “With REM…. you have a high degree of mental activity coupled with muscle atonia or paralysis,” he explains. “The processes that are optimized [for REM] require that kind of disconnection from the environment.”
Because the REM stage is when we dream, this temporary paralysis keeps us from acting out our dreams. We’re also less likely to be awakened by an outside noise during REM; instead, we may just incorporate the sound into our dreams, Grander notes.
REM sleep plays a key role in helping us regulate our emotions. It’s also important for creativity and creative problem solving, with our brains likely using this stage to build on the replay of memories that happen during the earlier stages.
While REM sleep accounts for roughly 25% of our total sleep time, the time we spend in this stage lengthens over the course of the night, with most of our REM sleep taking place during the second half.
How our sleep stages shift as we age
The amount of time we spend in various sleep stages changes over time. Infants, for example, go right into REM sleep and spend a greater portion of their total sleep time in REM sleep. They also have shorter overall cycles and multiple shorter periods of sleep, which gradually consolidate as they mature.
Compared to adults, children also spend more time in Stage 3 (slow-wave) sleep and have various qualitative differences in this stage as well.
Stage 3 sleep decreases during adolescence and continues to decline as we age: This change may start in the 20s or 30s, especially in men, Grandner notes. However, this doesn’t mean we need less sleep: The reduced time spent in Stage 3 sleep is offset by an increase in Stage 1 and Stage 2 sleep.
Our REM sleep time stays stable throughout adulthood (except in individuals with cognitive impairments, such as dementia).
Other things that affect stages of sleep
In addition to the changes that come with age, various other factors can also affect our sleep stages. This includes disorders such as sleep apnea, which is associated with reduced Stage 3 and REM sleep.
Substance use can also have an effect. “THC dramatically suppresses REM sleep,” Grander notes, while benzodiazepines generally suppress both Stage 3 and REM sleep. Alcohol affects the proportion of various sleep stages throughout the night and also can cause increased wakings during the second half of the night.
And then there’s the effect of sleep loss: If you’re getting just five to six hours of sleep, you’re likely cutting into both Stage 2 and REM sleep time, Grandner says. And if you’re getting less than five hours of sleep, you’ll be cutting into your Stage 3 sleep as well.
When you try to recoup this loss after a period of sleep deprivation, your recovery sleep will include a larger amount of Stage 3 sleep. It’s only after this stage is addressed that your REM sleep will likely also show a boost.
Which sleep stage is most important?
“There's no one stage of sleep that's the most important,” Grander says. “That's like saying ‘what nutrient is the most important?’”
Moreover, it’s important to remember that the amount of time we spend in each stage can vary quite a bit. The percentages noted don’t represent the amounts you should get, Grandner points out, “just the amounts that people often get.”
Although there are wearable devices to track and “optimize” sleep, you don’t actually need to track or even optimize your sleep.
If you do wear a tracker or use a non-wearable tracker, take note of your habits, diet, and activity levels on days when you wake up feeling especially well-rested, or on days when you feel you haven’t slept well. Over time, you can start to identify what improves or impairs your sleep.
If you’re concerned about your sleep, focus on whatever specific issues you may have and figure out “what’s preventing your body from being able to get the sleep it needs,” Grandner says.
Khosla echoes that advice. “The biggest hack we can do to improve our sleep,” she says, “is to get more sleep.”
