Each night you take a rollercoaster ride through the different phases of sleep. Though you’re unaware of what goes on while you’re sleeping, your brain and body are in an active state. Each stage of sleep has distinct restorative qualities, and how you move through each phase plays a large role in your body’s status the ensuing day.
During an ideal night’s sleep, you go through several 90-minute cycles that sample each phase of sleep. Each cycle plays an essential role in maintaining your mental and physical health. The amount of each phase of sleep can vary significantly between nights and individuals.
Stages of Sleep
Sleep has been traditionally divided into two categories: Non-rapid eye movement (NREM) and rapid eye movement (REM). Both are exactly what they sound like–your eyes either remain still or move rapidly under your eyelids. Together, these two types of sleep make up a single cycle where your brain progresses sequentially through each stage of sleep: wake, light sleep, deep sleep, REM, and repeat.
Stage 0: Wake
Awake time is the time spent in bed before and after falling asleep. It also includes brief awakenings during the night. These episodes are completely normal for healthy adults.
Stages 1 & 2: Light Sleep
Light sleep initiates your sleep cycle and acts as a transition to deeper sleep stages. During this stage your muscles begin to relax, your heart rate and breathing slow down, and you wake up easily.
During light sleep, you can expect the following:
- muscles relax and may jerk
- respiration slows
- heart rate decreases
- body temperature drops
- sleep begins
Stages 3 & 4: Deep Sleep
Deep sleep focuses on your body. It is the most rejuvenating and restorative sleep stage, promoting muscle growth and repair as well as waste removal in your brain. In this stage, you have difficulty waking up and are disoriented or groggy if awoken.
During deep sleep, you can expect the following:
- blood pressure drops
- blood flow increases to muscles
- repair hormones (i.e. growth hormone) are released
- tissue growth and cell repair occurs
- long, slow brain waves
- brain flushes out waste
Stage R: REM Sleep
REM sleep is essential to re-energizing your mind. REM is associated with dreaming, memory consolidation, learning, and problem solving. The time spent in this sleep stage usually decreases with age.
During REM sleep, you can expect the following:
- respiration increases
- heart rate increases
- temperature regulation is switched off
- brain activity is high; vivid dreams may occur
- body becomes immobile to stop you from acting out dreams
- blood flow increases to genitals
A Full Night’s Sleep
Your body cycles through these stages four to five times each night. Cycles earlier in the night tend to have more NREM sleep while later cycles have a higher proportion of REM. By the final cycle, your body may even skip NREM deep sleep entirely. Overall, your body spends more time in NREM phases of sleep.
Tips for Improved Sleep
All stages of sleep are important and your body naturally regulates your sleep cycles to make sure you get what you need.
Check out these patterns to see if your sleep is being disrupted:
- Increase in deep sleep after a hard workout: Exercise can increase your body’s prioritization of deep sleep the night after an intensive workout.
- Higher REM rebound after sleep deprivation: When you recover from a period of sleep deprivation, your body prioritizes deep sleep for the first few nights to repair your body and prepare for action. After several nights of sufficient deep sleep, REM sleep rebounds to focus on your brain.
- Interrupted sleep cycles after caffeine: Caffeine can increase the time it takes for you to fall asleep, cutting your sleep period short. Shorter sleep periods disproportionately cut down on your total REM sleep, as REM cycles are more likely to occur in later sleep cycles.
We all have those days when we “just need our coffee.” However, taking a look at your nightly patterns (e.g. heart rate, body temperature) and acting on your desire to improve your sleep can help you face those days well-rested.
- Stutz, Jan, Remo Eiholzer, and Christina M. Spengler. “Effects of evening exercise on sleep in healthy participants: A systematic review and meta-analysis.” Sports Medicine 49, no. 2 (2019): 269-287. (link)