Sleep Stages & Functions
Sleep is a naturally recurring state of our mind and body. The main characteristics of sleep are reduced consciousness, inhibited sensory activity, decreased ability to react to stimulus, and inhibition of almost all voluntary muscles (skeletal muscles). Sleep occurs in repeating periods and in two distinct modes: Rapid eye movement sleep (REM sleep) and non-rapid eye movement sleep (non-REM Sleep). While we are asleep most of our body’s system are in an anabolic state. During sleep our immune system, our nervous system, our bones and our muscles are repaired and restored. Our times of sleep and wakefulness are steered by our internal circadian clock (see Blog Posts “Our Body Clock Part 1”and “Our Body Clock Part 2”), which is linked to the light we are exposed to. Artificial blue light is known to interfere with our circadian rhythm, which can negatively affect our sleep quality and become a health hazard.
The most pronounced physiological changes during sleep happen in our brain, which uses significantly less energy during sleep than when awake. This is specially the case during non-REM sleep. Sleep increases our sensory threshold, which means we perceive less stimulus while asleep. During deep sleep we secrete bursts of growth hormones.
Non-rapid eye movement sleep
- Phase: The beginning of sleep and eye movement is slow. This phase may also be referred to as relaxed wakefulness. People in this phase often believe that they have been fully awake. In the process of falling asleep it is common to experience involuntary twitches.
- Phase: In this phase there is no more eye movement. Sleep is very light at this stage and the sleeping person can quite easily be awakened. In this phase our heart rate starts to slow down and our body temperature drops, as our body prepares itself for deep sleep.
- Phase: This is the deep sleep phase, also referred to as slow-wave sleep. While dreaming is more common in this phase than during the other non-REM phases, it’s not as common as during REM sleep. Research has shown that dreams in this non-REM phase, if they happen, occur most commonly during the morning hours. It is in this phase of sleep when parasomnias (e.g. sleepwalking) most commonly occur, which means the autonomous nervous system, cognitive process, and motor system are activated during sleep. Our body is now deep asleep and it’s much harder to wake someone up at this stage of sleep. If someone is woken up during deep sleep, the person can feel quite disorientated. During deep phases of non-REM sleep our body builds bone and muscles, strengthens our immune system, and repairs and regrows tissue.
As we grow older we sleep more lightly and get less of deep sleep and less sleep altogether. It is however scientifically proven that at an older age we need as much sleep as when we were younger.
Rapid eye-movement sleepREM sleep (also known as paradoxical sleep) is a unique sleep phase, distinguished by rapid movement of the eyes. During REM sleep our muscle tone in the whole body is low, which means our body is basically paralyzed (REM atonia), and there’s a tendency to dream vividly. REM sleep has always been closely associated with dreaming. The mental events which occur during REM commonly have dream hallmarks, including narrative and convincingness (resemblance to wakefulness). Sometimes they include elements of the dreaming person’s recent experience, taken from episodic memory. It’s estimated that 80% of all dreams happen during REM sleep.
The physiology of REM sleep is different from the other sleep phases. REM and non-REM sleep alternate within what’s called a sleep cycle and lasts around 90 minutes. As sleep cycles continue, or as we sleep longer, the proportion of REM sleep will increase. As we go through REM sleep periods, our eyes move rapidly, our brain activity is high, our heart rate goes up, and our breathing gets faster. During REM sleep our brain cleans out and removes neurotoxins, such as beta-amyloid, which is linked to Alzheimer’s disease.
During a night of sleep we usually go through four or five periods of REM. The relative amount of REM sleep varies considerably with age. While a new born baby spends more than 80% of the total sleep time in REM, adults typically spend about 20-25% of total sleep time in REM, or about 90-120 minutes per night’s sleep. As we grow older, we sleep more lightly and sleep less altogether. It is however scientifically proven that at an older age we need as much sleep as when we were younger.
REM sleep deprivation causes a significant increase in the number of attempts to go into REM stage while asleep. On so called recovery nights we will usually move to non-REM phase 3 and REM sleep quicker. We practically experience a REM rebound, which means we spend more time in REM sleep compared to normal nights. However, the rebound in REM sleep does not last as long as the initially missed REM periods, which means that we can’t really “make up” for lost sleep. Ongoing REM deprivation can result in anxiety, irritability, hallucinations and difficulties to concentrate.
Functions of sleep
Our organism physically restores and heals itself during sleep and removes metabolic wastes which build up during periods of activity. This restoration takes place mostly during deep sleep, when the body temperature, the heart rate, and the brain oxygen consumption decrease. The brain requires sleep for restoration, whereas for the rest of or body these processes can take place during quiescent waking. In both cases, the reduced rate of metabolism enables countervailing restorative processes.
When we are awake cells are damaged by our metabolism. While we are asleep restorative processes can take over, as metabolic rates and generation of reactive oxygen species is reduced. The brain actually removes metabolic wastes much faster when we are asleep than during an awake state. Sleep deprivation can disturb these restorative processes and affect our immune system.
While we are asleep, especially during REM sleep phases, we tend to have dreams. Dreams can be anything from entertaining to disturbing, or outright bizarre. Dreams can be very vivid and they can cause various emotions, such as happiness, sadness, fear or confusion. Some dreams may seem confusion, while others appear perfectly rational. We all dream, likely even several times every night, but often don’t remember it the next day. Think of dreams as images and stories our mind creates while we sleep. While dreams can occur anytime during our sleep, the most vivid ones usually happen during deep REM sleep, when our brain is most active. It is thought that the use of antidepressants, acetaminophen, ibuprofen or alcoholic drinks potentially suppress dreams, while melatonin may have the ability to encourage them.
Furthermore, it has been observed that sleep enhances our memory processing capabilities.
Awakening can either mean end of sleep, or just a moment to readjust the body position before falling back asleep. We typically awaken soon after the end of a REM phase or sometimes in the middle of REM sleep. During a night’s sleep a small amount of time is usually spent in a waking state. These days most of us wake up with an alarm clock. However, it is possible that we can also reliably wake ourselves up at a specific time with no need for an alarm clock. Many people sleep quite differently on workdays versus days off, which can lead to chronic circadian desynchronization. Another important factor in disrupting our circadian cycle is the impact of artificial blue light in the evening (see Blog Post “What is Blue Light?”).
How much sleep do we need?
While research cannot really define an exact amount of sleep people need, here are some recommendations for nightly sleep duration at different ages.
New borns (0-3 months): 14-17 hours
Infants (4-11 months): 12-15 hours
Toddlers (1-2 years): 11-14 hours
Pre-schoolers (3-5 years): 10-13 hours
School kids (6-13 years): 9-11 hours
Teenagers (14-17 years): 8-10 hours
Young adults (18-64 years): 7- 9 hours
Older adults (65+ years): 7- 8 hours
The exact amount of sleep needed varies from person to person. You’re best placed to assess your own sleep needs and habits.
Here are some simple yet effective healthy sleep tips:
Try to stick to a fix sleep schedule, even on weekends.
Practice a relaxed bedtime routine.
Exercise on a daily basis.
Pay attention to your bedroom surroundings, such as temperature, sound and light
Sleep on a comfortable mattress and pillows
Beware of the hidden sleep robbers alcohol and caffeine.
Turn off electronics before bedtime
Dim the lights, or ideally block out artificial lights, in particular blue light, up to 2 hours before bedtime.
Sublime Blue Blockers are very effective doing exactly that by blocking 99% of the artificial blue light.
Sleep is a very important pillar of our health and wellbeing and should be treated as a priority.
Have a great sleep tonight and always!