What Is Sleep?

Sleep is a natural, recurring state that supports health across the whole 24-hour day. During sleep, awareness of the outside world is reduced, but the sleeping brain remains active, and processes such as breathing, heart rate, hormone release, body temperature, metabolism, immune activity, and memory processing occur during sleeping hours. We need these regular periods of altered activity to restore, regulate and prepare for the next day.

Sleep is not only a human experience. Many animals sleep or show sleep-like rest states, including mammals, birds, reptiles, fish, and insects; dolphins and some other marine mammals can sleep with one side of the brain at a time. Even plants have roughly 24-hour circadian rhythms that help time growth and responses to light and dark.

In humans, sleep may look quiet from the outside, but it is not passive. Sleep interacts with and influences every body system, and vice versa. It also affects learning, memory, emotional regulation, attention, reaction time, and decision-making.

Once a person falls asleep, the body does not simply switch off. Sleep is an active state in which the brain and body change jobs. Some systems slow down to conserve energy. Other systems become more active so the body can repair, regulate, store information, and prepare for the next day. Sleep cycles through stages, each stage associated with important processes, including:

• The brain changes its activity
• The brain sorts and stores information
• The brain clears waste products (glymphatic drainage)
• The body shifts into repair and regulation processes
• The immune system increases activation and regulation
• The muscles relax through different sleep stages

Watch the video below to discover more about what happens when we sleep.

The two-process model describes two separate but connected processes that help regulate human sleep and wakefulness:

• Process S: the homeostatic sleep process, often described as Sleep pressure
• Process C: the Circadian process, the body’s roughly 24-hour timing system

These two processes work together. Process S tracks how long the body has been awake and how much pressure there is to sleep. Process C helps keep the body aligned with day and night, including when the body is most ready for alertness and when it is most ready for sleep.

Process S: sleep pressure

Process S is the pressure for sleep that builds the longer someone is awake. After a full night of restorative sleep, sleep pressure is usually low. From the moment a person wakes up, sleep pressure begins to build again.

One substance involved in this process is adenosine. While someone is awake, adenosine builds in the brain. Alongside the release of a hormone named melatonin, rising adenosine levels help signal that the body needs sleep. 

During sleep, the brain progressively clears away adenosine. Alongside a rise in a hormone called cortisol which increases arousal or wakefulness, the sleep pressure eventually reduces so that we can wake up again and become active.

Interestingly, understanding process S helps us understand how caffeine can affect sleep: caffeine temporarily blocks adenosine signals, making someone feel more alert even when sleep pressure is building. It takes many hours for the caffeine to stop blocking adenosine receptors and for sleep pressure to be felt again.

Process S helps explain the drive to sleep, but it does not work alone. A person can have high sleep pressure and still struggle to fall asleep if the circadian system is sending a strong alerting signal.

Process C: circadian timing

Process C is the body’s internal timing system. Circadian rhythms follow an approximately 24-hour cycle and help coordinate sleep and wakefulness, alongside other essential biological functions. Process C is driven by the body’s “master clock” – a small area of the brain called the suprachiasmatic nucleus, or SCN. The SCN receives information about light and darkness and helps coordinate important body signals and hormones to keep the circadian timing on track.

Process C plays a dynamic role over 24 hours. During the day, it helps oppose the rising pressure for sleep so a person can stay awake. In the evening, the process C alerting signals begin to decrease, allowing sleep pressure to have a stronger effect.

The sleep gate: when process S and process C align

Sleep becomes easier when Process S and Process C are aligned. When sleep pressure is high enough and the circadian system is no longer promoting alertness signals, the “sleep gate” opens. This is the window when falling asleep becomes easier. We perceive a sense of sleepiness and might notice some yawning and changes in our overall energy and alertness in the 1-2 hours prior to the “sleep gate” opening. This is the perfect time to do your wind-down routine, so that your brain and body are ready to enter sleep, and reap the benefits for the next day ahead.

If the two processes are misaligned, sleep can become harder. For example:

• A shift worker may need to sleep when the circadian system is in daytime awake mode
• A teenager has a natural tendency toward later sleep timing but must wake early for school
• Bright light exposure at night can delay melatonin release and push back sleep despite approaching bedtime
• Jet lag causes a mismatch between local time and the body’s internal clock

This is why sleep is not only about willpower or bedtime routines. The body needs both enough sleep pressure and the right circadian timing.