By Zhu Ying Ser
Although humans sleep for about a third of our lives, there is still a lot we don’t know about it. The complete mechanisms of sleep are still an enigma to biologists and neuroscientists around the world but researchers at Melbourne, Australia define sleep as reversible unconsciousness with particular brain wave patterns; sporadic eye movement; loss of muscle tone and sleep debt.
In order to better understand sleep, we must delve into the mechanics of circadian rhythms. A circadian rhythm is like a body clock that operates on a cycle that is roughly 24hrs and usually governs metabolic pathways within the organism. Although circadian rhythms are found in almost all living organisms, only the biological kingdom of animalia seem to require some form of sleep. In metazoans, the circadian rhythm dictates the sleep-wake cycle which can either induce sleep during sunlight or darkness.
Although there are many questions unanswered about sleep, we have come a long way from the ideas surrounding sleep in circa 400 BC. Greek physician Alcmaeon, suggested that sleep was an unconscious result of lack of blood flow to brain as it drains throughout the day. Others during this time thought that sleep was caused by the drop in surface temperature of the body. In 162 AD, the Ancient Roman physician Galen introduced the idea that consciousness resided in the brain as opposed to the heart. Despite this step in the right direction, little advances in understanding sleep were made until the 1900s; this was when neurons were discovered as units of the nervous system (Shepardn Jr et al., 2005).
Since this breakthrough, the role of the hypothalamus in the sleep-wake cycle; the function of hormones; the effects of sleep deprivation and the stages of sleep and its corresponding brain waves were unveiled. With current technology, the brain’s electrical activity can be measured through an electroencephalogram (EEG) test. EEG detects small fluctuations in the brain through small metal disc sensors that are placed on the scalp.
Sleep is categorised into two forms: rapid eye movement (REM) and non-rapid eye movement (NREM). REM sleep counts for a very small proportion of total sleep time and is associated with desynchronised and fast brain waves, eye movements, loss of muscle tone and suspension of homeostasis. A single sleep cycle is only 90 minutes long and alternates between REM and NREM, therefore there are 4-6 repeats of the cycle in a healthy night’s sleep (Macquet, 2005).
NREM sleep is further divided into 3 stages, the last one being slow-wave sleep (SWS). SWS is also known as deep sleep. Restoration of the body occurs during this deep sleep stage: body temperature, heart rate and brain oxygen consumption decreases. The slow-down of metabolic processes in the body reduces the rate of reactive oxygen species production which allows for restorative processes to take over. Anabolic hormones that induce growth are secreted during the restoration process which imply the importance of sleep for growth. Researchers at the Taylor & Francis group have found that without sleep, the body’s ability to heal wounds would deplete. It was found that rats that were sleep deprived were had compromised immune systems. It is suggested that this was due to the fact that sleep increase white blood cell counts. Other studies showed that depriving mice of sleep increases their chances of cancerous growths and reduced their immune systems’ ability to control cancers (Manzar & Hussain, 2012).
It is widely known that sleep plays a vital role in memory consolidation, but different types of memory are consolidated during different types of sleep. Memories travel through a specific pathway of encoding to short-term memory to long-term memory. Encoding is the process in which the memory is obtained: the sensory registers of the body receive the signals from environmental stimuli. Short-term memory (STM) has a small capacity and duration which means information that can be held in STM is limited and shortly forgotten.
An example of information that we would store in STM is a phone number that has just been recited. Long-term memory refers to a store which potential has no capacity limit nor duration limit. Consolidation of recent memories occur when new memories that have been encoded are reactivated which occurs during the SWS stage of NREM sleep. This reactivation of the memories allows them to become more long-term memories. Then in REM sleep, the newly transformed memories are further consolidated as long-term memories. Although investigation of memory processing in the past has been primarily focused on REM sleep, the involvement of NREM sleep is becoming more apparent (Astori, Wimmer & Lüthi, 2013).
There are many unanswered questions about sleep such as the reason as to why sleep deprivation is dangerous. We are aware that sleep deprivation can have fatal effects on living organisms but why it is so threatening is still not fully understood. Another sub context of sleep that is far from being fully understood is dreams, which many would argue is the best part about sleeping. Even with so many unsolved puzzles, the importance of sleep was always undebatable; however, this article has reiterated the justification of why many people would agree.