THE SCIENCE OF SLEEP
Experts say we should aim for at least seven hours of quality, uninterrupted sleep every night, but only 42% of U.S. adults are meeting this recommendation. We’re only recently discovering just how catastrophic this can be for our health, with sleep deprivation said to be linked to everything from weight gain and stress to heart disease and cancer.
But why is sleep so important, and what makes “quality” sleep? I’ll explore both of these questions in detail in my upcoming articles, but first, let’s look at the science behind sleep.
The sleep cycle
What we think of as sleep is actually a series of distinct states, which we cycle through several times per night. One sleep cycle lasts around 90-110 minutes and consists of four stages, which can be further classified as rapid eye movement (REM) and non-rapid eye movement sleep (NREM).
The first three stages of the sleep cycle are NREM sleep, where the muscles are relaxed and the brain is in deep, restorative rest. This is the most important type of sleep, essential for cognitive function, memory, and cell growth and repair.
The final stage is REM sleep, where the majority of your dreams take place. It’s known that REM is essential for nerve health in the brain, but many other theories about the purpose of REM exist. These include emotional processing, subconscious exploration, and memory encoding.
Stage 1: NREM1
This first stage is the transition between wakefulness and sleep. During this stage, your muscles are starting to relax and you feel drowsy, although your body and brain remain fairly active and you can still be woken very easily.
Stage 2: NREM2
You’re now in the second “light sleep” stage. Your heart rate and breathing have slowed down and your eyes have stopped moving, although you may still be easily woken. Your brain is experiencing short bursts of activity called sleep spindles. You might start to have dreams now, but they won’t be especially vivid yet.
Stage 3: NREM3
You’re now in deep sleep and cannot be woken easily. Your heart rate and blood pressure have slowed right down and your temperature has reached its lowest. You start to release Human Growth Hormone (HCG), which supports your cells as they grow, repair and regenerate. Your muscles are fully relaxed and are now working to replenish the energy used throughout the day.
Stage 4: Rapid Eye Movement (REM)
During the REM stage, your eyes move back and forth rapidly, hence the name. The brain is at its most active during REM sleep, but your body is paralyzed from the neck down (aside from essential functions like breathing). It’s thought that because your dreams are especially frequent and vivid during this stage, the paralysis is intended to prevent you from acting them out.
Sleep and circadian rhythms
Your sleep/wake cycle is one of a number of circadian rhythms, or 24-hour cycles of physiological activity, within your body. Each of these rhythms is governed by a central circadian clock in your brain, which takes its cues from external factors like daylight.
Other circadian rhythms determine essential bodily processes like eating, digestion, cell regeneration, and detoxification. In a healthy body, all of the rhythms work in synchronicity with each other around the 24-hour solar day. For example, you sleep when it’s dark, and you wake up when it’s light. You eat and digest during the day, and you rest and recover while you sleep. If one rhythm is disrupted, it can have a knock-on effect on the rest of the body’s rhythms.
The daylight cue
Your circadian clock “keeps time” via light receptors in your eyes. During daylight hours, you’re exposed to blue light, which tells your circadian clock to “wake up” your body. As the sun sets, the light shifts to red, which stimulates the release of melatonin and tells your central clock that it’s time to “power down” and rest.
Cortisol and melatonin
Cortisol and melatonin are the two most important hormones in the sleep/wake cycle.
You might know cortisol as the stress hormone. It’s one of the key hormones involved in your natural “fight-or-flight” response, giving you the energy and focus you need to take action in a life-threatening situation. It’s also present in lower levels during daylight hours, helping you wake up and stay alert throughout the day.
Melatonin, on the other hand, is responsible for the sleepy feeling you get later in the evening. During the 24-hour day, the two hormones work in opposition to each other, with one rising as the other falls.
What happens in the body during a 24-hour sleep/wake cycle?
During the early hours of the morning, as the sky begins to lighten, the receptors in your eyes detect an increase in blue light. This suppresses melatonin and triggers the release of cortisol, which slowly increases over the next few hours.
By sunrise, your cortisol levels are high and your body is ready to wake up. Your brain releases adrenaline and serotonin to help you become alert, and your blood pressure spikes in preparation for activity.
As your melatonin levels finally reach their lowest point during the morning, your cortisol levels reach their peak. You’re wide awake and alert, your heart and lungs are working harder, you’re digesting food, your metabolism is racing, and your body temperature is high.
After their morning peak, your cortisol levels steadily drop throughout the day. At sunrise, the light receptors in your eyes detect the shift to red light, and melatonin release is triggered to prepare you for sleep.
As the evening goes on, your heart rate and blood pressure start to drop, your breathing slows down, your muscles relax, and your temperature falls. By 11pm, cortisol is at its lowest and your digestive system and metabolism slow right down. You’re now likely to be feeling sleepy.
After midnight, your body enters the deepest phase of sleep, when essential restorative processes take place. The liver gets to work detoxifying the body, while the brain is busy consolidating the information you took in throughout the day. Your muscles are repairing and rebuilding themselves after a day of activity, and your cells are cleaning and replenishing themselves.
This activity peaks in the early hours of the morning as melatonin levels drop and cortisol levels start to rise. At sunrise, the cycle begins again.
When you realize just how many of the body’s most essential functions are facilitated by sleep, it’s easy to see why sleep deprivation is so disruptive to your health. And because these functions only happen during the deep sleep phase, you can still experience the effects of sleep deprivation after a full night’s sleep if you don’t spend enough of that time in deep sleep.