In the past, sleep was perceived as a passive state during which people lost consciousness for a couple of hours until they finally woke up again. This dated perception did not make any provision for meaningful brain activity during sleep or diverse sleep phases.
However, recent research reveals that the concept of passive sleep is far from accurate. Modern technology, which enables us to measure the electric currents of the brain and body, tells us that a host of activities take place while we sleep. It’s also common now to divide sleep into four distinct stages, between which we drift cyclically every night.
When you finally settle down for the night and let your thoughts wander, your body starts to relax, and you gradually dose off. This light sleep, balanced between consciousness and slumber, is the first sleep stage. If you were to wake up during this stage, there is a chance you wouldn’t think you were sleeping at all.
Stage 2 goes a little bit deeper but is still considered light. It is the most common stage. You will most likely spend the greater part of the night in this stage.
Stage 3 sleep is also known as deep sleep, slow-wave sleep, or delta sleep. During this stage, your body and brain are more detached from the outside world than during any other stage. This phase is generally considered to be the maintenance phase. The body clears out toxins and engages in general rest and restoration. Stage 3 sleep is the most difficult stage to wake up from.
REM is an abbreviation for Rapid Eye Movement. The name derives from the fact that our eyes move quickly in every direction while we are in this stage. REM sleep is also known as dream sleep because it is during this stage that we have the most vivid dreams and that our brains are most active. Our brain activity during REM sleep is almost identical to our brain activity when we are wide awake. Also, the body tends to be more active, and we experience elevated heart rates and irregular breathing during this stage. The brain and body are so busy during REM sleep that we have developed a mechanism to paralyze the largest muscle groups in our body so we don’t move in response to our REM brain activity. In other words, this is a way to protect you from acting out your dreams.
We drift cyclically between the different stages of sleep every night. The observation that we sleep in 90-minute cycles is mostly correct. However, the length of the cycles varies during the night, and there are individual differences. The distribution of stages across the night is different for every stage, such that you are likely to get most of your deep stage 3 sleep in the first third to half of the night, while you will get most of your REM sleep in the last third to half of the night. In general, your body will determine how long time you spend in each cycle. However, that percentage you spend in each stage can be significantly influenced by numerous factors, such as how much sleep you had the previous nights and medications you may be taking.