why do we need to sleep and dream?
This question about the function of sleep and dream has been studied for centuries. The oldest texts on these topics come from Ancient Egypt. There is obviously a lot to say about sleep, not only in Humans but in other species. This post aims to provide a foretaste and a brief overview of Sleep in Humans. Note that today, despite a growing set of experimental and clinical research outcomes, we are far from having a coherent and full picture of the function(s) of Sleep. We only start to realise its importance and benefits. However, Dream remains quite a mystery.
Ancient Egypt
In ancient times, Ancient Egypt had the most advanced medicine and surgery, and was definitely the place to be in terms of medical and surgery services. The Ancient Egyptian approach of sleep was highly spiritual, in accordance to their conception of Human, namely both material and spiritual to say it shortly. As such, we need to bear in mind that physicians were often priests. For more details about the medical professions and statuses in Ancient Egypt, see this chapter.
Thus, the medical papyri (see here for a list of ten main medical papyri that have been officially found), include diverse remedies to sleep disorders and snoring. Hypnagogic states were also induced by the use of anaesthetics and hypnosis (or trance like states). In that case, the visions reported by the patient served as a basis to determine the cure.
The Qenherkhepshef Papyrus, now owned by the British Museum, is the oldest text about dream interpretation. Dream was considered as a state of consciousness allowing to receive messages from gods or spiritual beings in general. This conception, although not recognised by the scientific community, has survived until modern days. Despite many speculations, the function and underlying mechanisms of dream are not understood very well.
Qenherkhepshef Papyrus (also known as Papyrus Chester Beatty III. Sheet 1) recto, columns 1-3: Hieratic dream book
Source: British Museum. More details here.
(Neuro)science of Sleep
Since sleep is characterised by an absence of behavioural response and an apparent extinction of consciousness, it has been commonly thought that sleep is needed for the nervous system / the brain. The first scientific studies of sleep, with the advent of Electroencephalography (EEG), focused on the description of brain activity during sleep. Brain activity during sleep is characterised by cycles (repeated over night), divided into mainly three stages: Light, Deep (also known as Slow Waves Sleep SWS), Rapid-Eye Movement (REM). Deep sleep stage tends to get shorter on the next cycle, while REM sleep stage tend to get longer.
During the Deep Sleep stage, metabolic activity is reduced of around 40%, and brain activity is at its lowest level. During the REM sleep stage, the metabolic activity level is similar to wakeful state, and so does EEG activity.
Dream has been associated to REM sleep since the discovery of that particular sleep stage by Aserinsky and Kleitman (1953). Dream and REM sleep have been held to be equivalent, as if the brain activity during REM sleep was directly causing the dream experience. Yet, this was more an assumption than a fact, typical of contemporary neurosciences that presuppose the equivalence between neuronal activity and conscious mental activity. Studies has provided clear evidence (See Foulkes, 1962, Oudiette et al, 2012, Carr and Somolonova, 2019) that this assumption is false. Dreams are also reported outside REM sleep. The neurophysiological activity may impact the nature of the contents of the dream, the memorisation or simply the recall.
Sleep is triggered by mechanisms situated in the nervous system, but actually has a wider physiological function. Sleep is needed for the whole body, for all cells. Furthermore, not only the quantity of sleep matters, but also the regularity and duration of each stage as they appear to have different functions and their disruption may impact health differently.
I am preparing a lecture on Sleep so I will only brush the global picture and leave the details for later. The reader interested in an overview of the metabolic function of sleep can read this paper, and this one for the impact of sleep deprivation on neurological disorders.
Sleep deprivation and fragmentation have been associated with endocrine, metabolic, and immune dysregulation. As we all know, sleep also impacts brain activity and particularly mood and emotional regulation, pain sensation threshold, reward and risk perception, attention and memory performance during wakefulness.
A very recent line of research (see for example Sulli et al, 2019) has shown that disruption of circadian clock (the 24-hour internal clock in our brain that regulates cycles of alertness and sleepiness by responding to light changes in our environment) is associated with cancer. In other words, working at night or late shift professions are more likely to develop cancer. The integration of circadian biology into cancer research is hoped to provide new insights to improve our quality of life, prevent cancer or making treatments more effective.
The importance and benefits of Sleep for the physiological health, including mental health, are more and more studied, known and recognised. However the function of dream, that was associated with the REM stage is less clear.
The physiology underlying dreaming activity is quite misunderstood and this lack of understanding feeds the spiritual conception of dream function. I hope to do so for my next lecture on Sleep and Dream.
