Light sleep makes up most of an adult’s sleep cycle. We spend about 50% of our sleep in light sleep. Although not as restorative as deep sleep (also called slow-wave sleep) and not as vivid in dreams and emotions as REM sleep, it is an important part of our night’s rest.
During light stages of sleep, the memory is consolidated, the brain produces sleep spindles (which seem to have numerous important roles), and the body prepares for other sleep stages.
Light sleep as a part of a typical sleep cycle
There are four sleep stages – one rapid eye movement (REM) sleep phase, and three Non-REM stages. Non-REM (or NREM) sleep is made up of light sleep (stages 1 and 2) and deep sleep (stage 3).
A typical sleep cycle looks like this: stage 1 – stage 2 – stage 3 – stage 2 – REM – stage 2.
Here we see that light sleep comes as soon as we fall asleep and before and after deep and REM sleep. Not every sleep cycle is the same – in the first half of the night, our sleep consists of frequent and long periods of deep, restorative sleep, whereas in the second half, REM takes over.
Still, light sleep makes the most of our night’s rest. However, sleep cycle can significantly differ in those who suffer from sleep disorders like narcolepsy – a condition in which a person might fall asleep directly into REM sleep, or those diagnosed with obstructive sleep apnea (OSA), who wake up many times a night due to impaired breathing. Frequent wakings result in the person spending too much time in light sleep.
What are light sleep stages and what characterizes them?
Light sleep stages are stage 1 and stage 2. The first sleep stage happens only after wakefulness or awakening, and it might not be seen as ‘real sleep’ by some. The reason is that, in stage 1, people are still aware of their surroundings as they slowly drift off. We are also very easy to wake up.
Stage 1 lasts one to ten minutes and, if woken up from stage 1, people usually deny being asleep. Even so, there are some psychological and physiological changes that occur in the body.
- Breathing changes – it slows down
- Heart rate becomes regular and blood pressure drops
- Core body temperature slightly drops
- The brain produces alpha and theta waves
What could perhaps best convince a denying sleeper that he/she was was actually asleep is a typical dream which occurs at the onset of sleep. In stage 1, most people have a dream of or feel that they are falling down, as they suddenly jerk their bodies into wakefulness. This quick movement is called the hypnic jerk.
After stage 1, the brain cycles into stage 2 light sleep. Stage 2 is longer – it lasts for up to twenty minutes. As it prepares the body for deeper sleep, we are more ‘detached’ from the surroundings and more difficult to wake up. Other stage 2 characteristics include:
- Relaxed muscles
- Metabolic processes slow down
- Breathing and heart rate slow down more than in the previous sleep phase
- Body temperature and blood pressure decrease
- Memory is consolidated – especially muscle memory
- The brain produces waves of high-frequency bursts known as sleep spindles (sleep spindles persist through deep sleep)
During stage 2 we are able to dream, although those dreams are not coherent, vivid and do not have a narrative like those in REM sleep.
Motor aspects of our memory are transferred (consolidated) from short-term into long-term memory. The brain revisits information and strengthens brain-muscle pathways. These motor aspects are called muscle memory. It is a type of procedural memory – memory which is all about how things are done. If we are learning how to juggle, the motor sequence will be repeated in dreams, which significantly helps with the process of learning.
Babies spend most of their time in REM and deep sleep, but as we age, we have more and more light sleep. Adults of advanced age have more light sleep than young adults.
What is the function of sleep spindles?
Sleep spindles are brainwaves which come in bursts; they occur in stages 2 and 3. They have many roles, but scientists assume there are many more yet to be discovered.
- Allowing tight sleep. People who sleep tighter have more sleep spindles. These spindle brain waves are linked to distortion of information in the brain’s auditory or visual cortex – so if there’s a soft sound or a slight change in light, we will not be aroused. Those with fewer spindles have lighter sleep and wake up easily. Spindles may be making the main difference between heavy sleepers and light sleepers.
- Memory consolidation. A study has shown that the number of spindles increases if: during a task of motor learning an odor is released, and the same odor repeated in sleep stage 2, after subjects fall asleep. Not only do spindles increase as a result, but the memory of how to perform the activity is much better.
Spindles don’t only strengthen the memory – they also help us forget the unnecessary bits, so our brain is ‘decluttered’ form the excessive and unnecessary information.
- Brain and nerve development in babies and fetuses. Spindles seem to help the brain learn how the body is mapped – which nerve pathways lead to which muscles.
There’s a link between our chronotype (sleep-wake time preference) and sleep spindles – in early birds, weaker spindles were measured compared to afternoon and evening types.
Waking up from stage 2 is quite easy, but we can’t say the same for deep sleep – when someone wakes us up from deep sleep, we are drowsy, disoriented, and might suffer from a long-lasting sleep inertia.
Naps are the most effective if they are about 20 minutes short or 90 minutes long. The reason is this – in the first 20, or even 30 minutes, you are unlikely to reach deep sleep, so you can only experience light, refreshing sleep, from which you can wake up easily. It is also known as a ‘power nap’. It can be really helpful if you are having a long, exhausting day. However, short light sleep doesn’t help much if you’ve been sleep deprived for some time.
The second nap recommendation is 90 minutes. This is the time which an average person typically needs to go through one sleep cycle and return to light sleep. A long nap includes deep and REM sleep and is more restorative than the short one. Deep sleep will decrease your overall need for sleep (sleep propensity) so you might sleep less that evening, or go to bed later than usual.
If you are completely exhausted in the evening and want a nap, you should definitely limit the nap time to under 20 minutes. Longer sleep in the evening is highly likely to have an effect on your sleep pattern.
Can I be deprived of light sleep?
Generally, people are not deprived of light sleep stages – we reach them as soon as we fall asleep. We can be overall sleep-deprived if we’re not sleeping at all.
There are situations in which, although a person sleeps for eight hours, is still tired all the time. This is known as non-restorative sleep – it usually happens if someone can’t reach REM or deep sleep for some reason (drugs, stress, alcohol, sleep disorders). They spend almost all night in light sleep.
Therefore, light sleep is something we get every night, regardless of the other factors. However, there is a number of behavioral and health aspects which can prevent us from reaching other sleep stages.
Having only light sleep is poor quality sleep. A good night’s sleep is when the brain cycles through all the stages.
Light sleep and sleep deprivation
If we are continuously deprived of sleep (or some sleep stages), we feel tired, moody, are unable to concentrate or focus. Our immune system gets weaker and overall health poorer.
When a person is deprived only of the REM stage or deep sleep stage, an interesting thing happens. Once they are given the chance to sleep (without the disrupting factors like drugs or awakenings), the brain goes ‘fast forward’ through light sleep, and then gets into a prolonged period of the previously deprived stage. This is known as REM sleep rebound / deep sleep rebound.
Just like we tend to sleep longer after being deprived of sleep altogether, we tend to ‘make up’ for the individual sleep stages. However, this kind of sleep cuts back on light sleep, which is also important, as mentioned above.
You can easily make up for the sleep you’ve lost the previous night, but it’s not reasonable to try to counter long-term sleep deprivation only with long sleep hours. If there are other health consequences, they need to be addressed accordingly (for example, weight and insulin resistance problems which occurred as a consequence of the lack of sleep have to be treated with a proper diet or prescribed medical treatment).
- Laventure S, Fogel S, et al. NREM2 and Sleep Spindles Are Instrumental to the Consolidation of Motor Sequence Memories. PLoS biology. March 31, 2016. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4816304/ Accessed February 20, 2019.
- Sleep, Learning, and Memory. Healthy Sleep. Harvard. http://healthysleep.med.harvard.edu/healthy/matters/benefits-of-sleep/learning-memory Accessed February 20, 2019.
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