Sleep Cycles and How They Work
While a good night’s sleep can certainly leave us feeling refreshed and ready for the day – and a bad night’s sleep can leave us feeling worse for wear – it’s perhaps oversimplifying matters to say there is such a thing as ‘good sleep’ and ‘bad sleep’.
Sleep can indeed provide many benefits, but to see how sleep affects the restorative processes of the mind and body, we must look beyond the concept of sleep as a whole. Instead, we must look at sleep in terms of cycles, and the way in which those cycles can be broken down into stages.
Each sleep cycle contains a number of sleep stages, which include non-REM sleep, made up of light sleep and deep sleep stages, and REM sleep. Moving through these stages of sleep, or sleep patterns, allows our body to rest and restore energy levels.
While it may have been previously thought of as a passive process that ‘shuts down’ the body, sleep is in fact an active state. Affecting both physical and mental wellbeing, sleep – in its various stages – works to help the body cope with stress, recover from illness, and solve problems.
So, what are the stages of sleep within a sleep cycle and what do they do?
The period of non-REM sleep is made up of stages 1 to 4. Progressing through these four stages, we move from very light sleep during stage 1 to very deep sleep in stage 4. Through each stage of non-REM sleep, there is very little muscle activity, but all muscles retain their ability to function.
Shortly after falling asleep, the body moves into stage 1 of the non-REM sleep cycle, where the brain starts to produce alpha and theta waves, and the heartbeat, breathing and eye movements slow down. As a kind of introduction to sleep, this stage is relatively brief, lasting only several minutes. As this is a very light stage of sleep, someone in stage 1 of sleep is still somewhat alert and can easily be woken.
While this stage of sleep is also fairly light, stage 2 sees the brain produce a sudden increase in brain wave frequency known as sleep spindles. After this, brain waves slow down, preparing for deeper sleep. The heartbeat and breathing slows, muscles relax, body temperature drops and eye movements stop. More time is spent in stage 2 than any of the other stages of sleep.
Stages 3 & 4
Stage 3 is the beginning of deep sleep, where the brain begins producing slower delta waves. With no eye movement or muscle activity, this is the point where the body becomes less responsive to outside stimuli and is harder to be awakened. In Stage 4, the body moves into the more intense level of deep sleep. It is during these deep sleep stages that the body repairs muscle and tissue, boosts immune function, stimulates growth and development, and builds up energy for the next day.
If deep sleep is about the body, REM sleep is about the brain. During a REM cycle, the brain is very active and the body is very inactive, and it is at this point when most dreaming occurs. Vivid dreams accompany rapid eye movement, and an increase in heart rate, blood pressure and breathing.
Playing an important role in learning and memory function, REM sleep is the time in which the brain consolidates and processes information from the day before so that it can be stored in long-term memory.
Sleep Cycle Timing
During sleep, the body cycles through these stages, with each cycle lasting around 90 minutes. But just as each cycle time can vary – from around 50 minutes to 100 minutes and more – the timing of each cycle also varies. The body spends an unequal amount of time in each sleep stage, according to its needs.
Over the course of the night, the amount of time spent in each sleep stage shifts. During the first half of the night, the body concentrates on deep sleep, and in the second half of the night, more REM sleep occurs.
The complexity of sleep doesn’t end there. Non-REM sleep tends to occur in the earlier hours of the night – no matter what time sleep actually starts – while REM sleep is more likely to occur in the early morning. That means those who go to bed later may miss out on important deep sleep and its restorative processes.
Changes in Sleep Over a Lifetime
Sleep changes as we get older. The percentage of REM sleep is highest during infancy and early childhood, then, during adolescence and young adulthood, the percentage of REM sleep declines. As we move further in to old age, the percentage decreases further. The amount of non-REM sleep also decreases with age, so people under age 30 may have about two hours of restorative sleep every night, while those over 65 might get only half an hour.
Sleep Stages and Their Effects
When it comes to deep sleep, there is no way to get too much of it, as the body has its own natural drive for it. Once that need has been met, the body will progress to light sleep and REM sleep. Not enough deep sleep, however, will prevent the body from following its restorative processes, leaving you feeling unrefreshed.
While there is no real way to get too much or too little light sleep – as this is more of a ‘filler sleep’ the body progresses through between deep sleep and REM sleep – it is possible to get too much and too little REM sleep.
Too little REM sleep can leave you feeling groggy, less able to focus, and possibly with memory problems. Too much REM sleep, on the other hand, can lead to too much brain activation, leaving you irritable and angry, potentially exacerbating symptoms of depression and anxiety.