Sleep, mental health and memory

How we think and feel not only affects our waking lives, but also has a significant impact on our sleep, especially our ability to initiate and maintain the sleeping states.

There has long been an established link between a large number of mental health conditions and poor sleep. In fact, one would be hard pressed to find anyone living with an enduring mental health condition who does not also have a coexisting sleep problem.

Mental health conditions and insomnia almost universally seem to come hand-in-hand, with insomnia being a core diagnostic feature of many mental health conditions (e.g. major unipolar depression, bipolar depression, Lewy Body Dementia and so on).

We also have good evidence for effective co-treatment (particularly for depression with co-morbid insomnia) (Manber et al.,2008), and emerging evidence for the co-treatment other conditions, e.g. people living with chronic pain and co-morbid insomnia (Tang, 2008).

There is an emerging school of thought that a large number of mental health conditions (notably those which tend to be acquired (e.g. depression and anxiety) rather than those to which people are more genetically predisposed (e.g. type 1 schizophrenia) are regarded as disorders of memory.

To expand, and taking depression as an example of this, we know that people with depression are prone to certain thought processes that are involved in the development and maintenance of the condition. These are referred to as 'cognitive bias' and 'selective attention'.

Depressed people will tend to focus on negative situations or stimuli more than on more positive or neutral situations or stimuli (selective attention), and they will tend to see situations as more desperate then people who do not suffer from depressive symptomatology (cognitive bias).

These two phenomena interact and, over time, can contribute to the maintenance and increasing severity of a depressive episode; and so lead an individual into a persistently and deeply depressed state of mind – major, clinical, unipolar depression. This process is almost always accompanied by poor sleep and, as mentioned above, insomnia is a core diagnostic feature of major depression.

Our mental health is driven by how we remember (and forget) things that happen to us in our everyday lives, and sleep is critical in these processes of the 'memory' and the 'forgettery'.

There is a very small part of the brain, right in the centre near the hypothalamus, called the hippocampus. When we are in deep sleep the brain is quiet and acquiescing, with the

exception of the hippocampus, which is fully awake, up and running, firing on all cylinders, and at its most active in the 24-hour cycle.

We know from a large number of studies that the hippocampus is critically important for the consolidation of memory, acting as a control centre in the middle of the brain, sending messages out into the cortices above it.

Taxi drivers whom have learned all the streets in London, referred to as 'doing the knowledge' have been shown to have larger hippocampi when training when compared to other people who have not had this training (Maguire et al., 2000). Chronic insomniacs have also been shown to have smaller hippocampi those of normal sleepers (Riemann et al., 2009).

These findings – and many others – have identified deep sleep as critical for the consolidation of memory. There is a useful analogy that can explain this a bit more. If you imagine that the inside of your head is a busy office, and that every thought that you have during the day generates a piece of paper in that office.

During the course of a day we have many, many thoughts, about everything and anything. Some of those things are important, or even very important (e.g. I need to call the mortgage adviser), but other thoughts may be less so (e.g. which socks shall I wear today?; Ooh - look at that dirty car). But each of these generates a piece of paper. Then we sleep and our secretary (the hippocampus) comes into the office and starts to organise things.

This organisation consists of sorting through all the paper and throwing out the rubbish (socks / dirty car); and prioritising the important things (sticking the "call the mortgage advisor" piece of paper on top of the in-tray for tomorrow morning) i.e. encoding this into the cortex as something to be remembered and not forgotten.

If we sleep well we will do a good job of organising our office and it will be tidy again by the morning, if we do not sleep well then the office will be a mess in the morning.

Dr David R. Lee and Sleep Unlimited have partnered with the BPS to produce a new professional development online learning series on psychology and sleep - available now on BPS Learn.