This is often referred to as paradoxical sleep and for good reason. From one perspective, this is the deepest stage of sleep because you are unresponsive to stimuli and your muscles are completely paralysed but it would be a mistake to think that this is because you are completely ‘out for the count’.
During REM sleep your brainwaves will fall within the ranges associated with theta, alpha and even beta waves indicating brain activity which is equal to that experienced during the waking period. This is the part of the night where you dream, and the afore-mentioned paradox.
Being that you are not conscious, and so acting out your dreams with physical movement would be damaging, an area of your brainstem called the sublaterodorsal nucleus becomes active during REM sleep. This acts to inhibit skeletal motor neurons effectively shutting off your brain’s ability to communicate with the wider body other than areas needed for necessary function (such as your heart and lungs). Because it’s not likely that eye movements are going to cause you to hurt yourself, they have also been spared this inhibition and as you dream, your eyes are free to flicker around at will. This has been displayed elegantly in trials such as that conducted by Aserinsky, because the times of night where brain activity is at its highest (suggesting dreaming) correlate precisely to the time of night which involves jerky eye movements (as measured by an electrooculograph, which can detect activity in the muscles that control eye movements).
It’s not only brain activity which increases at this time. In contrast to other sleep states where your body generally slows down, your blood pressure, heart rate, breathing and metabolism will increase during this period to almost waking levels. Sexual arousal is also common with penile and clitoral erection being present (in fact REM sleep-related penile erections are one of the diagnostic tools used to determine whether erectile dysfunction is physiologically or psychologically rooted). You will likely experience short periods of wakefulness during REM sleep, too, which you are unlikely to remember. If you are over-stimulated (as we will get to later) you could wake up fully at this time, potentially then taking a long time to get back to sleep.
During brain development REM sleep appears critical. Newborns spend about 50% of their sleeping time in REM sleep, which drops to around 30% at 3 months of age and 20% by 6 months. This is associated with the maturation of your cerebral cortex (the more advanced and ‘human’ part of the brain) and in fact rats deprived of REM sleep during development show various neural difficulties as they age. Not only this, but infants will fall straight into REM sleep rather than following a typical adult sleeping pattern which will be explained in the next section. At age 2 REM accounts for 20-25% of sleep; a fact which lasts until adulthood. Once you reach adulthood your REM sleep percentage will drop by about 0.6% per decade of life. In short during childhood you may spend 8 hours per day in REM sleep, during early adulthood that will be around 2 hours and by age 70 it’ll be approximately 45 minutes.
REM sleep is activated due to a release of 2 neurotransmitters – acetylcholine and serotonin in the pons area of your brainstem (a band of nerves which includes the abovementioned sublaterodorsal nucleus), evidenced by the fact that removal of this area eliminates REM sleep entirely. This means that REM sleep, as for sleep in general, is not a state which you fall in to when things deactivate, but an actively ‘caused’ state.
As you can see, REM and non-REM sleep are hardly equivalent states and sleep itself is a hugely important part of development, but why do we need it as adults?
While deep, stage 3 sleep is necessary for rejuvenation and feeling refreshed after sleep, a lack of REM sleep (as can be caused by SSRI antidepressants and other medications) doesn’t appear to have any obvious negative effects to day-to-day function. However, a lack of REM sleep does seem to impair your ability to learn complex tasks and remember things (possibly one reason it’s so important during infancy – you have a lot to learn) and while we don’t completely understand why, REM sleep seems important to your brain because you will compensate for a lack of it. If you have a reduced amount of REM sleep for 1 night, you will experience REM rebound sleep the following night involving REM sleep occurring earlier in the sleep cycle and for longer.
With that said, to close this section we need to explain the full state of affairs with REM sleep. Those who have complete REM sleep deprivation, as mentioned, do not experience short-term negative effects and (because of the rejuvenating nature of stage 3 sleep) may not even realise they haven’t had ‘normal’ sleep. Additionally, these individuals may dream more in stage 3 sleep to make up for their missed opportunity. REM sleep seems to be important, but we aren’t totally sure why, and it seems that your body is able to at least partially make up for any lost ground if REM sleep is absent.
During this section, we have discussed the 4 stages of sleep and mentioned sleep cycles, so let’s explore that before we circle back to one important question – what’s the point of sleep?