Sleep is a difficult thing to define. It doesn’t fit easily in any box – which only adds to its great enigma.
Until recently, sleep was seen as a passive, almost dormant state of rest. While rest is clearly a crucial part of sleep, we now know that on a cellular level, the brain is highly active during sleep in processing experiences and memories from the day, as well as preparing the mind and body for the pressures of the day to come. Beyond neurological functions, sleep is also vital in the physical restoration of the body.
While considerably more is known now about sleep than decades before, most of what we know comes from studying those who do not sleep enough. Sleep can be as intangible and ungraspable to researchers as it can sometimes be to us.
Sleep itself is not a single state. Broadly speaking, sleep is split into two stages – one is associated with brisk fluttering eye movements, and so-named rapid eye movement (REM) sleep. The other is not associated with such movements and is so creatively named non-rapid eye movement (NREM) sleep. NREM is further subdivided into NREM 1 and 2, considered light sleep, and NREM 3, which is deep sleep. Each stage has its own characteristic functions.
NREM 1 sleep is the shallowest form of sleep. It’s the sort of sleep you have when you watch TV on the sofa in the evening after a busy day. Sometimes you’re aware you’re dipping in and out of it; other times, you may be convinced you haven’t slept. Your breathing slows, your temperature drops and your mind and body rest to a moderate level. NREM 1 is the first stage of sleep you enter when you sleep in your bed at night, making up around 5% of your night’s sleep. Following NREM 1 is NREM 2, a slightly more intense form of light sleep, which makes up around 45% of your night.
Following NREM 2 is usually NREM 3. This is the good stuff. This is the deep restorative state of sleep your body needs to experience to feel refreshed in the morning. Most of your body’s growth and repair occurs in this stage, which is helped by having most of your growth hormone secreted by your pituitary gland during deep NREM. Your immune system, large parts of which are protein based in the form of antibodies, is maintained during this phase. The physical relaxation, lower blood pressure and heart rates associated with NREM give vital respite to your cardiovascular system. While this may not be noticed on a night-to-night basis, this nightly respite significantly boosts cardiovascular longevity over the course of decades. Deep NREM 3 sleep usually makes up about 20% of the night.
The brain has its own cleaning system called the glymphatic system, which is mainly active during NREM. This system removes a lot of waste proteins that accumulate due to the waking activities of the brain.
Neurologically, NREM is needed to solidify your memories, the list learning type that you might need before an exam, and the physical learning required for tasks like learning to drive a car.
When we try to learn something, the memories go first into short-term memory in a part of the brain called the hippocampus. This has a limited capacity and is a less reliable place to store memories. Sleep moves such memories from the hippocampus to the longer-term storage of the cerebral cortex – a process called consolidation. This process also empties the hippocampus so more learning can occur the next day.
Various experiments have demonstrated this link between memory and sleep. One such study asked two groups to learn a list of names. One group were allowed to sleep while the other stayed awake. Of course, the group who slept did significantly better on checking recall.
REM sleep is the phase of sleep where the most vivid dreaming occurs. These dreams are so vivid that the body must be temporarily paralysed; otherwise, it would act out the dreams. REM is thought to be necessary for emotional regulation, as well as the consolidation of memories. Some theories suggest that dreaming is a place to pilot our new memories or experience strong emotions to practice regulating them.
Each sleep stage is visited multiple times in a reasonably predictable sequence called a sleep cycle. A cycle lasts, on average, around 90 minutes. After one cycle, the body wakes up entirely (though you may not be aware of this) before entering a new sleep cycle.
There are various theories on why we wake like this. One suggestion is that humans are very vulnerable to predators during an eight-hour block of sleep, so waking periodically allows us to re-evaluate the safety of the environment we are sleeping in and note any predators moving in. One group of people who are very aware of our tendency to wake after each sleep cycles are new parents. Children move through sleep stages in a slightly clunky fashion. At around four months old, children wake after each 90-minute cycle and frequently struggle to go back to sleep. A phenomenon called four-month sleep regression, and it can be very tough to deal with!
The below chart is called a hypnogram which shows the usual nightly route through sleep stages. This is known as your sleep architecture and shows a couple of key things.
One is that the bulk of your deep NREM 3 sleep occurs in the first half of the night, and most of your REM takes place nearer the end of the night. This explains why when you need to catch up on your sleep, people generally find going to sleep earlier is more effective than lying in, as to get more deep NREM 3. It is also why the most bizarre and vivid dreaming you experience takes place just before your alarm goes off.
The majority of what we know about sleep comes from researching people who are sleep deprived. Very few body systems escape unscathed from poor sleep, so the effects can be extensive.
We all know how our mood can be affected by a night of poor sleep – often low, irritable and annoyed. A large part of this relates to the lack of REM and its benefits in emotional regulation. When insufficient sleep happens for weeks or months, it is associated with developing several mental health conditions, including depression and anxiety.
Good evidence links habitual poor sleep with the development of a variety of significant physical health conditions, including high blood pressure, heart disease, stroke and diabetes.
As well as causing illness, poor sleep can affect your health and safety in other ways. Without enough sleep, it isn’t easy to focus and concentrate. One scientific journal estimates 15-20% of car crashes are related to inadequate sleep. Another study looked at the impact of sleep deprivation on driver vigilance levels via various tests, including hazard perception. A period of prolonged wakefulness of 19 hours had a more significant effect on driver vigilance than being at the drink driving limit of alcohol. While most of us wouldn’t dream of driving over the legal alcohol limit, how many of us think twice about driving when we haven’t slept enough?
Sleep is a universal requirement. There are few things that can affect so many parts of your life as sleep, from cardiovascular health to your immune system, to memory, concentration and your safety on the roads. Fortunately, there are many ways to improve your sleep quality and quantity.
Visit www.thebettersleepclinic.co.uk or www.theinsomniacentre.co.uk for information concerning sleep disorders or to book a consultation.
