What is sleep and why is it important?

In the 1920s, Hans Berger first used the electroencephalogram (EEG) to measure brain wave activity in humans. For the first time, we could record the electrical activity of the brain during sleep and wake. Scientists came to realise that sleep was not a uniform or passive process but that it changed through the night.

Today, a further two measures of sleep are typically assessed together with EEG. Muscle activity during sleep is measured using electromyography (EMG) and eye movements are measured using electrooculography (EOG). This three part assessment of sleep is usually referred to as polysomnography (PSG).

An overnight sleep study is not always needed to diagnose sleep problems

While these measures have allowed us to learn much about the structure and function of sleep, and you can read more about this below, these tools are not always needed to diagnose common sleep problems.

Insomnia is one of the most common sleep disorders that does not require a complex sleep assessment to diagnose. Insomnia can be diagnosed by reviewing sleep diaries and discussing symptoms (which can be done through Sleepstation).

There are two distinct states of sleep

• Rapid Eye Movement (REM) sleep
• Non-REM sleep

There are two basic types of sleep: rapid eye movement (REM) sleep and non-REM sleep (which has three different stages). Each is linked to specific brain waves and neuronal activity.

REM sleep is the phase of sleep in which most dreaming occurs

Intense dreaming occurs during REM sleep as a result of increased brain activity, but paralysis occurs simultaneously in the major voluntary muscle groups. REM is a mixture of brain states of excitement alongside muscular immobility. For this reason, it is sometimes called ‘paradoxical sleep’.

NREM sleep is relatively dreamless sleep

During NREM, the brain waves on the EEG recording are typically slow and of high voltage, the breathing and heart rate are slow and regular, the blood pressure is low, and the sleeper is relatively still.

You cycle through all stages of NREM and REM sleep several times during a typical night, with increasingly longer, deeper REM periods occurring toward morning.

Dream-like events that happen during NREM are much shorter than those we experience during REM sleep and are more likely to to concentrate on a single emotion.

There are three stages of NREM sleep

NREM sleep is divided into three stages; N1, N2, N3, each of increasing depth. (Note you may encounter mention of stage 4 sleep in older texts that is because before 2005 NREM sleep was divided into 4 stages of sleep but stages 3 and 4 were then combined into Stage N3).

You cycle through all stages of NREM and REM sleep several times during a typical night

During the night, you pass through the four sleep stages: N1, N2, N3, and REM sleep in what are known as ‘sleep cycles’. Sleep progresses cyclically from N1 through to REM. Each sleep cycle lasts approximately 90 to 110 minutes in adults. The first couple of sleep cycles have long periods of uninterrupted stage N3, which is also known as Slow Wave Sleep (SWS), with relatively short REM periods. Later in the night, the REM periods lengthen and SWS is mostly absent. Thus, the first third of the night is predominantly SWS sleep, and the latter part of the night is spent in the lighter stages: N2 and REM sleep.

We all wake during the night and that’s normal

Contrary to popular belief, good sleepers don’t ‘sleep through’ the night. At the end of each sleep cycle we all become fully alert for a short time. Most people won’t remember waking up, but we all do. When we become more aware of these awakenings, and start to worry about them, they can become problematic. If you’re very aware that you’ve been awake for long periods during the night then you might have a sleep problem. Find out if we can help you sleep better, by completing a short questionnaire here.

N1 is the lightest stage of sleep

Stage N1 (1-5% of sleep) is the lightest stage of sleep and is the transition between wake and sleep. You enter sleep via stage N1, whenever you fall asleep. It is the type of sleep you experience when you feel like you are drifting in and out of sleep, particularly at the start of the night.

We aren’t always consciously aware we’re in N1 sleep

When you are in stage N1 sleep, you can be wakened easily. If you are awakened, you will probably claim not to have been asleep as we’re often not consciously aware of this stage of sleep.

The transition from wake to sleep can be ‘bumpy’

During the transition from wake to sleep, many people experience sudden muscle contractions or ‘jerks’, a sensation of falling or a ‘presence’, benign or otherwise, in the room. Falling asleep is not like switching off a light bulb; there are a number of complex processes that need to occur, and these, so-called ‘hypnagogic’ events seem to be glitches in the process of entering sleep. Although they may be perceived as worrying or scary, they are in fact normal and harmless.

We spend almost half the night in N2 sleep but we don’t know why

Stage N2 accounts for approximately 45-50% of sleep and, although it is the most substantial single portion of sleep, it is the stage which we know least about. It is known to play a part in memory, but we do not yet entirely understand why we spend half the night in this stage.

We spend about 1/4 of the night in ‘deep’ N3 sleep

As sleep becomes deeper, slow brain waves (called delta waves) start to appear, and we enter N3 or SWS (20-25% of sleep). N3 is the deepest stage of sleep, and when someone is in SWS, it can be very hard to wake them. SWS is believed to be most closely linked with the restorative processes of sleep and is thus the part of sleep that makes you feel like you have had a good sleep. It also plays a crucial role in making you feel well-rested and energetic during the day. SWS is vital for memory and learning and it is for this reason that children have proportionally more SWS than adults.

Physical growth occurs during N3 (slow wave sleep)

One other reason why children have more slow wave sleep (SWS) than adults is believed to be because it is the only stage of sleep in which we physically grow. It is also during SWS that some people, particularly children, experience behaviours known as ‘parasomnias’ such as bedwetting, sleep talking, sleepwalking or night terrors.

Regardless of sleep need, the amount of slow wave sleep we get remains fairly stable

Both short and long sleepers essentially have the same amount of SWS. Scientists believe that this indicates that a minimum amount of N3 deep sleep is needed per night, no matter how long the total sleep time. Given the apparent importance of SWS, it is not unsurprising that, after one night of partial or complete sleep deprivation, the brain attempts to make up all the missed SWS, causing us to experience a feeling of sleeping more soundly following a period of sleep deprivation.

We spend up to 1/4 of the night in REM sleep

During Rapid Eye Movement (REM) sleep (20-25% of sleep) the eyes can be observed jerking rapidly back and forth under closed eyelids, hence its name. It is during REM sleep that most of our ‘story-like’ dreams occur. Dream-like events can occur in any stage of sleep, but dreams that happen outside of REM sleep are generally thought to be shorter, more focused on a single emotion and lacking the narrative complexity of dreams in REM sleep.

It is during REM sleep that emotional memories are processed

REM sleep is involved in processing emotional memories and ensuring our psychological health. During REM, our brainwave activity can increase to levels experienced when a person is awake; breathing becomes more rapid, irregular and shallow, heart rate increases and blood pressure rises. In order that we do not act out our dreams, we lose muscle tone during REM and thus we are effectively paralysed. It is most likely that we will wake up during REM sleep.

If we don’t get enough sleep our physical and mental health suffers

Individual sleep need is like height - we are all different, and it is, to a large degree, genetically determined. Anywhere between four and eleven hours can be considered normal, but getting just one hour less sleep a night than you need can have measurable effects on your physical and mental health and cumulative lack of sleep can have long-term health consequences.

If you feel sleepy during the day, then you are probably not, for whatever reason, getting the sleep you need during the night.

Summary of key points:

• Sleep problems aren’t always diagnosed in a sleep lab
• There are two distinct states of sleep, REM & NREM
• There are 3 stages of NREM sleep, N1, N2 & N3
• We dream mostly during REM sleep
• We cycle through all stages of sleep several times during the night
• We all wake during the night and that’s normal
• If we don’t get enough sleep our physical and mental health suffers