We spend about a third of our lives asleep, yet many of us don't understand what happens during those hours. Sleep is not a uniform state of being. Instead, we cycle through two main types of sleep: Non-Rapid Eye Movement (NREM) and Rapid Eye Movement (REM) sleep. A full sleep cycle, which includes stages of both, typically lasts about 90 to 110 minutes. We usually go through four to six of these cycles each night.
The Stages of NREM Sleep
NREM sleep is divided into three stages, progressing from light to deep sleep. This is the period when your body repairs tissues, builds bone and muscle, and strengthens the immune system.
- Stage 1 (N1): This is the initial "dozing off" stage, lasting just a few minutes. Your heartbeat, breathing, and eye movements slow down, and your muscles relax. It's the lightest stage of sleep, and you can be easily awakened.
- Stage 2 (N2): This is a period of light sleep before you enter deeper sleep. Your heartbeat and breathing slow down further, and your body temperature drops. Eye movements stop. We spend the most time in this stage—about 50% of our total sleep time. This stage is critical for memory consolidation.
- Stage 3 (N3): This is the deep sleep stage, also known as slow-wave sleep. It's much harder to wake someone up from this stage. Your heartbeat and breathing are at their slowest rates. This is the most restorative stage of sleep, essential for physical recovery, growth, and immune function. We get more N3 sleep in the first half of the night.
The Stage of REM Sleep
After passing through the NREM stages, you enter REM sleep. The first REM period of the night usually occurs about 90 minutes after you fall asleep and is relatively short. As the night progresses, REM stages get longer, with the longest periods happening towards the morning.
During REM sleep, your brain becomes more active. Your breathing becomes faster and irregular, and your heart rate and blood pressure increase to near waking levels. As the name suggests, your eyes move rapidly behind your eyelids. This is the stage where most vivid dreaming occurs. REM sleep is crucial for cognitive functions like learning, memory consolidation, and emotional regulation. Your body's muscles are temporarily paralyzed during this stage, which prevents you from acting out your dreams.
Why Sleep Cycles Matter
Understanding your sleep cycle can help you improve your sleep quality. For instance, waking up at the end of a sleep cycle (during a lighter stage of sleep) generally feels much better than being jolted awake from deep N3 or REM sleep. This is why you might feel groggy even after a long night's sleep if your alarm goes off at the wrong point in your cycle.
Factors like age, stress, alcohol, and caffeine can disrupt the natural progression of these cycles, reducing the amount of restorative deep sleep and REM sleep you get. To protect your sleep cycles, it's important to practice good sleep hygiene: maintain a consistent sleep schedule, create a relaxing bedtime routine, and ensure your bedroom is dark, quiet, and cool.
By respecting the intricate architecture of your sleep, you can take meaningful steps towards not just getting more sleep, but getting better quality sleep, which is fundamental to your overall health and daily performance.