Sleep can feel like simply switching off. In reality, it is an active, carefully organised process that affects almost every system in the body. In our new series, Sleep Science, we explain what science says about sleep and how to sleep better. We begin with the basics: how sleep works.
Two systems that control sleep
Scientists often describe sleep as being regulated by two main processes working together.
1. Your body clock (circadian rhythm)
Your body has an internal clock that runs on a cycle of roughly 24 hours. It is controlled by a small area of the brain called the suprachiasmatic nucleus, in the hypothalamus. This clock influences when you feel alert or sleepy, along with body temperature, hormone release and digestion.
Light is the strongest signal that keeps this clock in time. Morning light helps set it, while darkness in the evening allows the body to release melatonin, a hormone that signals it is night.
2. Sleep pressure
The longer you are awake, the more "sleep pressure" builds. One key chemical involved is adenosine, which accumulates in the brain during waking hours and makes you feel sleepy. During sleep, adenosine levels fall again. Caffeine works partly by blocking adenosine's effects, which is why it can make you feel more alert (more on this in Part 4).
When your body clock signals night and sleep pressure is high, falling asleep is usually easiest.
The stages of sleep
Sleep is divided into two main types: non-rapid eye movement (NREM) sleep and rapid eye movement (REM) sleep.
NREM sleep
- Stage N1 (light sleep): the transition from wakefulness, lasting a few minutes. You can be woken easily.
- Stage N2: a deeper light sleep where heart rate slows and body temperature drops. Adults spend about half the night in this stage.
- Stage N3 (deep or slow-wave sleep): the deepest sleep, when it is hardest to wake up. It is important for feeling physically restored and is linked to growth hormone release and immune function.
REM sleep
During REM sleep, the brain becomes very active, the eyes move quickly behind closed lids and most vivid dreaming happens. Most muscles are temporarily relaxed, which stops us acting out our dreams. REM sleep is thought to support learning, memory and emotional processing.
Sleep cycles
Through the night, you move through these stages in cycles lasting roughly 90 minutes, though this varies from person to person. Most adults go through four to six cycles a night.
Early in the night, cycles contain more deep NREM sleep. Later, towards morning, REM periods become longer. That is one reason why cutting sleep short in the morning can reduce REM sleep in particular.
Brief awakenings between cycles are normal. Most people do not remember them.
What sleep does for you
Research links healthy sleep to many benefits, including:
- Memory and learning: sleep helps strengthen memories formed during the day.
- Mood: poor sleep is associated with irritability and difficulty managing emotions.
- Physical recovery: tissues are repaired, and energy is restored.
- Immune health: regular, sufficient sleep supports the immune system.
- Brain "cleaning": studies suggest that during sleep the brain clears some waste products more efficiently.
- Metabolism and heart health: long-term short sleep is associated with higher risk of several health problems.
Key takeaways
- Your body clock and sleep pressure work together to decide when you feel sleepy.
- Sleep moves through light, deep and REM stages in roughly 90-minute cycles.
- Each stage plays a role in how you feel and function the next day.
In Part 2, we look at how much sleep people really need at different ages.
Sleep Science: the full series
- Part 1: How Sleep Works (you are here)
- Part 2: How Much Sleep You Need
- Part 3: Better Sleep Habits
- Part 4: Caffeine, Alcohol and Food
- Part 5: Screens, Light and Melatonin
- Part 6: Insomnia
- Part 7: Snoring and Sleep Apnoea
- Part 8: Naps, Shift Work and Jet Lag
This article is for general information only and is not medical advice. If you have ongoing sleep problems, loud snoring, breathing pauses or daytime sleepiness, please speak to a doctor.
