The psychology of sleep

Hi everyone, this week’s post follows on from last week’s, which was all about our body clock (if you’d like to read it then click here). First, I will look at the stages of sleep and the link between sleep and memory, before moving on to talk about a sleep disorder – narcolepsy.

Scientists have been able to identify the different stages of sleep by using brain imaging methods such as EEG. This measures brain potentials from the scalp to establish brain activity. Five stages of sleep have been identified – these are shown in the sleep cycle diagram shown below.

sleep-cycle

I’m sure you’ll have heard of the 5th stage – Rapid Eye Movement, or REM sleep. During this stage of sleep, all motor movement is stopped to prevent the body from acting out our dreams.

REM sleep has also been linked to memory consolidation involving our hippocampus. This is a structure in the centre of our brain which has been found to be important for long term memory. It is thought that when we are asleep, the hippocampus is involved in replaying previously activated cells, which establishes our long term memories.

Tilley & Empson (1978) tested the hypothesis that REM sleep is involved in memory consolidation by testing participants’ recall of a story after deprived stage 4 sleep, or deprived REM sleep. They found that participants who had disrupted REM sleep had significantly poorer recall than participants who had disrupted stage 4 sleep.

The involvement of sleep in helping learning has also been shown by neuroimaging studies. Maquet et al (2003) measured patterns of brain activity of participants while they practiced a reaction time motor task. They then measured their brain activity while they were asleep, and found the same patterns of activity appeared during REM sleep. Interestingly, the amount of reactivation of these patterns correlated with the extent of learning, which suggests sleep is vital for consolidating our memories and aiding learning.

Narcolepsy:

This is a sleep disorder which affects 0.5-1% of the population, and is characterised by excessive sleepiness. Patients’ sleep cycles are disrupted, and they have fast entry into REM sleep (Vogel, 1976). Sudden sleepiness is usually brought on by exciting or emotionally charged events, and is often accompanied by a loss of muscle tone, called cataplexy. It is thought to be an intrusion of REM sleep into wakefulness, which is shown by one of the symptoms – sleep paralysis. Narcolepsy  can be caused by a lack of a chemical called orexin, which regulates sleep (NHS Choices). This reduction in orexin has been hypothesised to be caused by cells in the immune system mistakenly attacking the cells that produce orexin, however this has not been proven.

I leave you with the video our lecturer showed us when we learnt about this topic – a dog with narcolepsy. Enjoy – 

I hope you liked this post and don’t forget to leave any requests for future posts in the comments!

The science behind our body clock

Hi everyone, this week’s post is about something we talk about a lot – our body clock. Even though everyone’s heard of it, I’m guessing it’s not something people are too familiar with, so I’ll try and explain it here.

The 24 hour cycle we live in accordance with is known as a Circadian Rhythm. This is set according to our environments – in particular in response to the amount of light or darkness. Therefore, the eyes are really important in regulating our circadian rhythms – people tend to forget that our eyes have functions other than sight!

The signals from the retina on the back of the eyeball travel towards the brain via ganglion cells – specialised cells which carry signals containing the visual output. These ganglion cells project towards a structure called the Suprachiasmatic Nucleus (which is fortunately abbreviated to SCN!) in the hypothalmus, which is shown in the image below:

circadian_rhythm_labeled1

The SCN is known as the body’s ‘master clock’, and is responsible for keeping us in time with the circadian rhythm. Studies have shown that if this structure is damaged, then people’s circadian rhythms are virtually abolished. Cohen & Albers (1991) carried out a case study of a 34 year old woman who had part of her hypothalamus taken out as part of an operation to remove a brain tumour. Although the surgery successfully removed the tumour, she was left with a condition called hypersomnolence, which is basically excessive sleepiness in which patients can fall asleep at any time, although the extra sleep does not help reduce the symptoms. This shows the importance of the SCN in regulating our sleep pattern, and how disorientated we would be without it.

I hope you found this post interesting – check back next Thursday for my next blog post all about sleep.