Dreams are made of this

Sleep is such a natural part of life and yet it remains largely unexplained by science. Leila Sattary finds out what dreams are made of and why sleep is so hard to understand

Most people think they do not get enough of it. Dolphins do it half a brain at a time. Rats die after three weeks without it. Sleep is a common feature in the animal kingdom and yet it remains one of the greatest unsolved mysteries in science.

Sleep is not simply about resting the body and mind. Humans can rest for long periods of time but we eventually need to sleep if we want to perform at full capacity. There are a number of theories to explain our need for sleep – brain ‘maintenance’, memory consolidation or healing the damage done by daily stress endured while awake. Some of these theories are more developed than others and it is clear that the mystery of sleep cannot be solved by neuroscience alone as the key may lie with evolutionary factors.

Why did animals, and even insects, evolve to need sleep? Sleeping creatures are vulnerable to predators and unable to eat, procreate or perform any other basic survival activities. Most animals sleep on a daily basis but yearly cycles like hibernation of mammals in the winter or migration, where birds fly for days without stopping, are obvious deviations in the pattern.

Recent research has showed that almost all animals sleep as do many insects. Even plants have ‘dormant’ states though they do not have nervous systems or brains. This has challenged the idea that sleep is some sort of rejuvenation time for the brain.

The evolutionary argument for why we need sleep hinges on a cost-benefit analysis. While awake, animals can eat, care for young, keep watch for predators but these activities use high levels of energy. When asleep, we reduce our body and brain metabolism while retaining a high level of responsiveness to the environment. Other survival benefits of sleeping include a reduced risk of injury or detection by predators. Maybe eight hours a day is the best evolutionary balance for humans? This could explain why we need less sleep as we grow old. We sleep longer and deeper when we are young because we have a high metabolic rate that is reduced during sleep and we have others to keep watch. When we are older we have slower metabolic rates and are relied on to protect the young from danger and so sleep does not optimise our energy usage to the same extent.

Surely this neat evolutionary explanation is not the whole story – it does not address the mysterious dreams we experience while asleep. When we are asleep we cycle between two states called ‘rapid eye movement’ or REM and non-REM sleep. While in non-REM sleep brain activity is low and consistent across the brain. REM sleep is frantic and very similar to when we are awake and characterised by dreams, rapid eye movement and muscle paralysis to stop us acting out our dreams. These phases of sleep are also experienced by reptiles, birds and other mammals so it likely arose early in evolution.

One theory is that non-REM sleep is used for organising thoughts related to the events of the waking day – replenishing proteins, strengthening synapses and inserting receptors – you need to be unconscious so no neuronal activity gets in the way. The brain then flips into REM mode to test out the new connections. This ‘create and test’ process would explain why the brain cycles through the phases and why sleep always begins with non-REM sleep.

However, many scientists believe there is more to sleep than just evolution, or giving time for the brain to organise its thoughts. There is evidence that REM sleep is involved with learning how to perform complex tasks like driving a car or learning the violin. Experiments show that people who are taught a particular task and then allowed a good night’s sleep improve significantly at the task the following morning in comparison to those who are prevented from REM sleep. Is REM sleep consolidating their learning?

One thing is clear – there is far to go before we fully understand the meaning of sleep and dreams. I find it fascinating that in REM sleep the brain ignores the obvious in favour for the crazy and unexpected. This could simply be based on our minds beta-testing our new connections. However, I wonder if there is more to it – is sleep the origin of creativity?