Sleep stages
During sleep we pass through five stages: non-REM 1, 2, 3 and 4 as well as REM (rapid eye movement) sleep.
These stages progress cyclically from non-REM 1 through to REM then begin again with stage 1, noting though that not all stages are reached during each cycle. A complete sleep cycle takes an average of 90 to 110 minutes, with each stage lasting between 5 to 15 minutes.
In order to experience good restorative sleep we need to complete four to five of these cycles. The cycles are not the same throughout the night; the early cycles have more deep sleep (physiological recovery) whereas the REM phase (psychological recovery) is progressively longer with each cycle.
Follow the black line to see how we transition from one stage to the next across five cycles
Non-REM stage 1 is the transition phase between wakefulness and sleep. During this stage we start to feel sleepy, our eyes may burn a bit and start to feel heavy. Muscle tone reduces and if we are sitting down our head might "bob around". This stage is very short (5 to 10 minutes) and when you awaken someone during this stage they might report they were not really asleep.
Non-REM stage 2 is the light sleep stage where we become less aware of our surroundings but are still relatively easy to wake. Our body goes into a state of deep relaxation and starts to prepare itself for deep, restorative sleep. This stage plays an important role in memory, even more so than REM sleep.
Interestingly, during sleep memories are being literally transferred from one part of the brain (hippocampus where we store memories during the day) to another part of the brain (cortex where long-term memories reside). During sleep the brain, selectively, replays what it got up to during the day to facilitate this process and consolidates our memories.
Non-REM stages three and four (deep sleep) are associated with physiological recovery. During deep sleep our brain is rather inactive and we are difficult to wake; our temperature, heart rate, blood pressure and oxygen consumption goes down - our body and its many systems is repairing, recovering, removing waste through our lymphatic system, boosting our immune function and getting us ready for a new day.
Deep sleep is not exclusively about the recovery of our body however as during this time the removal of metabolic waste from our brain also takes place. Our brain cells shrink by about 60%, creating space for cerebrospinal fluid to flow past our brain cells, taking waste that has build up during the day with it. This waste removal also takes place during the day but it is twice as fast during sleep as during our awake hours. This effectiveness of this brain cleaning has a big influence on reducing the risk of neurological disorders (for example Alzheimer’s disease) and the overall aging of the brain.
Deep sleep occurs more during the earlier part of the night and as such disruptions to sleep in the earlier part of the night (e.g. as a result of alcohol, taking the stress of the day into your sleep, imbalances in your autonomic nervous system) can quickly start to impair your physiological recovery and therefore your health, energy levels and whether or not you wake up feeling rested.
REM sleep occurs later in the night and is linked to extremely vivid dreaming and psychological recovery, including functions such as processing information from the day, forming new memories, stimulating the central nervous system and restoring brain chemistry to a normal balance. During REM sleep, our brain activity is quite high (close to being awake) but most of our muscles are immobilised (to avoid us acting out our dreams!). We are easy to wake and can recall our dreams.
Note that for adults it is actually quite normal to wake up a few times during the night, during these REM sleep periods. More often than not we are not actually that aware we have briefly woken up but some people do notice this. Provided your fall back asleep relatively quickly, this does not constitute a sleep problem and is a normal part of your sleep cycles.
The amount of time in these phases, and therefore the quality of your sleep and your brain and body's ability to recover, decreases with age.
