What is sleep
Sleep a fundamental part of human existence. Yet, is has been very difficult to pin down an exact definition of sleep itself. Sleep can be defined as a naturally recurring state of mind and body. Our nerve cells control whether we are awake or asleep by regulating the release of certain neurotransmitters (chemicals that nerve cells use to communicate with each other). When we fall asleep neurons appear to switch off the signals that keep us awake. There are usually five different phases of sleep: stages 1,2,3,4 and REM (rapid eye movement sleep). You can find out more about the different sleep stages here.
Why do we sleep
Even though we spend one third of our lives asleep, researchers are still attempting to understand why exactly humans and all other animals need sleep. From an evolutionary perspective it seems startling that we spend so much time in a state, where we can’t protect ourselves, forage for food or procreate.
We do know that sleep is necessary for survival. Studies with rats have shown that complete sleep deprival leads to death within only 3 weeks. Some further research suggests that a lack of sleep can negatively impact the immune system. It seems that sleep is necessary for us to function normally. If we don’t sleep enough this has lots of effects on our day-to-day functioning, such as making as drowsy, unable to concentrate, and worse at remembering things.
Several theories to date have attempted to explain the functionality of sleep. It is generally thought that sleep allows our bodies, especially our brains, to recover. Some people believe that sleep gives nerve cells a chance to shut down and repair themselves. It is thought that without sleep, nerve cells become depleted of energy or polluted by cellular byproducts, which impairs their normal functioning. Additionally, sleep seems to give the brain the opportunity to strengthen or get rid of connections between nerve cells based on experiences.
A new role for sleep: waste clearance
Exciting new research by Professor Maiken Nedergaard from the University of Rochester Medical Centre in New York has revealed a whole new function for sleep. In 2013 her group published work that had been carried out in rats and showed a system of microscopic fluid-filled channels that removes waste chemicals from the brain mainly when the brain is shut off (i.e. during sleep or anaesthesia).
During the day the brain is continuously active and producing lots of different proteins. These accumulate in the brain and need to be cleared away. It seems that clearance of waste products only occurs at rest- i.e. during sleep or when under anaesthesia.
“You can think of it like having a house party. You can either entertain the guests or clean up the house, but you can’t really do both at the same time.”
Professor Nedergaard and her group have named this newly identified system, which is yet to be proven to be present in humans, the glymphatic system.
Sleep and dementia
Ever since the invention of the light bulb, many people stay up later and get less sleep. Studies in the U.S. have shown that a significant proportion of the population is getting less than 6 hours sleep every night. However, it is not only sleep duration that has suffered- many people also have worse sleep quality. For example, reading electronic books before going to bed has been shown to affect the time it takes to fall asleep, to reduce melatonin (the hormone that regulates the body’s internal body clock) levels, and to reduce alertness in the morning.
The importance of sleep has become evident in relation to a number of neurologic disorders, including dementia. It has been shown that as the brain ages, the glymphatic system functions less efficiently. Interestingly, many elderly people have disturbed sleeping patterns, especially those who may be suffering from dementia.
Experiments showed that mice with genetic mutations in the APP and PS1 genes (both of which have been implicated in causing Alzheimer’s disease) have reduced influx through the glymphatic system, as well as reduced clearance of the amyloid beta protein. Similarly, wild type mice that were pretreated with amyloid beta showed reduced functioning of the glymphatic system. This suggests that while Alzheimer’s disease likely leads to accumulation of amyloid beta and reduced clearance through the glymphatic system, the aggregation of amyloid beta will feed forward to further slow down the glymphatic system. Hence, there is an additive effect that is likely detrimental to the functioning of this waste clearance system. Professor Nedergaard and her colleague have written a detailed review of the glymphatic nervous system and its role in disease of the central nervous system.
There are currently no therapies that directly address the functioning of the glymphatic system. However, researchers are hopeful that this may provide a new avenue to intervene in these disease processes.