The Stages of Sleep: What Happens During Each

Most people have heard of REM or NREM sleep. Fewer know how each stage of sleep contributes to well being, or how sleep stages work together. In this guide, you’ll learn the stages of sleep, how they contribute to the recovery of your brain and body, and why “optimizing” for good overall sleep is likely to give you better results than “REM hacking” or “deep sleep hacking”.

What are sleep cycles?

Sleep cycles are routines in which the brain and body transition from lighter to deeper sleep, and then to a dream-rich state called REM. Each cycle lasts about 90 minutes, and most people need 4-6 of these cycles per night to be well rested. The brain and body coordinate during each cycle to stimulate and relax muscles, dilate vessels and clean out metabolic waste, rehearse the learnings from the day in the form of repetitive brain activity, and create dreams.  You can think of a night of sleep like a few cycles of a washing machine programmed back-to-back. With each cycle, the brain and body become a little cleaner, your memories a little stronger, and you are a little more refreshed for the day to come.

What are the sleep stages, what do they do, and why are they important?

Sleep cycles are a sequence of 4 predictable stages: 3 stages of NREM sleep, followed by 1 stage of REM sleep. The “n” stands for non, and the “REM” stands for rapid eye movement. Let’s take the process of NREM sleep first, because this is the first entry point into healthy, typical sleep.

NREM

NREM Stage 1: Dozing Off

What is NREM Stage 1 Sleep, and What is it For? NREM Stage 1 is the process of dozing off. Dozing off is a transition state between wake and sleep, and should take up to 5 or 10 minutes. In this stage, your muscles begin to relax, your eyes stay closed, and you may experience an occasional feeling of falling (also called a hypnic jerk or a sleep start). During this process, heart rate and breathing slow,  HRV rises, and alpha brain waves slow into beta and theta waves. Additionally, your body begins to shuttle blood from central organs to the skin, resulting in decreasing core temperature and increasing skin temperature - especially at the hands and feet! Stage 1 makes up ~5-8% of your night.

The amazing thing about stage 1 sleep is that, despite these changes in physiological state, you remain somewhat aware of your surroundings. If someone wakes you during this stage, you may even recall your surroundings and conversations or report that you were awake the whole time!

Tips for Dozing Off Peacefully

• Wind down without screen time. This probably won’t come as a surprise, but more smartphone use before bed can increase the amount of time it takes to fall asleep. Cutting off tech-use at least 30 minutes before bed, and using blue light blockers might help. But note that both sensitivity to light, and the stress associated with scrolling vary from person to person. Don’t be afraid to allow yourself a longer tech-free wind down - you deserve it.

Fun Fact: Men tend to spend more time in N1 than women. The speed at which we doze off has to do with how fast we can dilate blood vessels and shunt heat from our core to our skin, a process that may be aided by estrogen in women of reproductive age.

NREM Stage 2 or Light Sleep

What is Light Sleep and What is it For?  Light sleep prepares the body and mind for entry into deeper sleep, and begins to reinforce learnings of the day. During this stage, brain activity begins to show interesting shapes by alternating patterns of slow waves and fast, jerky patterns called sleep spindles and k complexes. These patterns contribute to the growth of new connections or synapses in the brain, and to memory consolidation. Sleep spindles may also help “shut down” your senses so you won’t wake up. In the body, muscles alternate between further relaxation and small bouts of movement. Eye movements stop. Heart and breathing rate continue to slow, whereas HRV and skin temperature continue to rise.

Fun Fact: This is the stage in which most teeth grinding occurs. As you progress through subsequent sleep cycles, you might not spend much time in this stage. If you wake up (e.g., to use the restroom), you may drop briefly into light sleep when you return to bed.

NREM Stage 3: Deep or Slow Wave Sleep

What is Deep Sleep, and What is it For? Deep sleep is the final portion of NREM. It helps the body repair tissues via growth hormone, remove metabolic waste, grow bone and muscle, and improve immune function. If that weren’t enough, it also contributes to memory - and even creativity! Flushing out metabolic waste products is achieved via carefully coordinated dilation of vessels in the brain and body (a phenomenon that will go even further in the brain during REM sleep!).

During this stage, the surface of the brain produces memory-boosting, sweeping waves, called delta activity or slow waves.  The muscles do not move, and trends continue in heart rate and breathing rate reduction. HRV and skin temperature* continue to increase.

Tips for Good Deep Sleep

• Make a couple vigorous exercise sessions part of your weekly routine.
• Warming the skin at bedtime, even about half a degree centigrade, may increase time in deep sleep.
• Drink a lot of coffee? Do you think your body has adapted to it, but you are still experiencing  suboptimal sleep? Try reducing or eliminating caffeine in the 8-12 hours prior to sleep (yes, it can take that long to get out of your system). The recommended “dose” of caffeine is 400mg or less and to stop drinking it around 1pm. If you need a drastic cut back, make sure to talk to your doctor as sudden drops in caffeine can cause headaches and other physical symptoms.
  

Fun Fact: Have you ever wondered why some days you can wake up before your alarm, and other days the blare drags you foggily into the morning? You may be waking up during different parts of your sleep cycle. Some parts of the cycle (e.g., deep sleep) produce grogginess when interrupted, whereas waking from REM or light sleep do not. If you do shake awake someone in deep sleep, they are likely to feel disoriented for a few minutes, and be cognitively impaired for about an hour! Our bodies naturally wake up after the completion of a cycle - that is, at the transition from REM to light sleep.

REM Sleep

What is REM Sleep and What is it For? REM sleep is characterized by active, wake-like beta wave activity on the surface of the brain, slower theta waves in the hippocampus (a memory and spatial organization center of the brain), greater incidence of dreaming, and suppression of muscular activity in the body. During REM, heart rate and breathing increase, while HRV and skin temperature decrease. Researchers are still figuring out the functions of REM and dreaming, but consistently report REM’s importance for brain development, emotional regulation, and memory. REM also helps consolidate spatial (how did I drive to get there?) and contextual memories (what was the vibe of that dinner party last night?).

Finally, REM contributes to the process of clearing out waste product buildup in the brain. You can think of both NREM and REM contributing to the “rinse”  cycle of brain and body, and of both as essential for proper rest. We can’t function well with yesterday’s “gunk” clogging up the pipes - whether those pipes are in our brain or our bodies. REM bouts increase in duration across the night, meaning early morning awakening can cut your REM short. For more, see our article on REM sleep.

Tips for Good REM Sleep

• Avoid early alarm clocks. Because REM sleep occurs preferentially in the morning hours, it’s likely to get cut off if you set a 5:30 or 6:00am alarm. A stable sleep schedule - and, for most of us, an early bedtime - is needed to set up the brain and body for adequate REM.  Irregular bedtimes paired with consistent alarm times contribute to overall sleep disruption, shorter sleep duration, and cutting short both deep and REM bouts (e.g., bedtime between 10-12, and alarm time consistently at 6:30). If you can't omit an alarm from your routine all together, then consistent bed time may be even more important for you. In fact, a recent study of middle aged women found that more irregular bedtimes contributed to the development of insulin resistance.
  

Fun Fact

• REM sleep duration decreases by about half in men over the age of 50 as compared to young men, and the reduction is associated with greater evening cortisol levels. This also happens right in the thick of andropause, when testosterone levels are falling, and when the circadian clock predisposes people to earlier awakening. Routine and providing an adequate, low stress environment for good sleep is even more important as we age.

Sleep Stages Work Together: Optimize Sleep Timing, Wind Down, and Natural Wakeup for a Full Night’s Rest

“All of these cortical rhythms [e.g., delta waves, sleep spindles] have, individually, but mostly through their coordination with other hippocampal and cortical patterns, been related to memory consolidation” - Girardeau and Lopes-dos-Santos, 2021, Science

We commonly get the question “How can I improve my REM sleep?” or “How can I improve my deep sleep?”. The best way to improve one part of your sleep cycle is to focus on improving overall sleep. Hacking one sleep stage in response to feeling tired is similar to re-doing your wardrobe by purchasing only left shoes. Sleep stages (and outfits!) work well we understand the details but focus on the big picture. Just like a good outfit takes a top, bottom, and shoes - good sleep takes adequate relaxation to prepare you for deep sleep, and sufficient time in bed to enable the longer REM bouts of the early morning. For best sleep, we recommend you understand your sleep stages and focus your attention on behaviors and strategies that optimize your overall rest.

To learn more about how sleep stages are measured - see our Ultimate Guide to Sleep Wearables.