Chemical theory of sleepOne of the oldest theories about why people get sleepy is that a fatigue or toxin substance gets accumulated in the body. Initially, it was thought that this substance should reside in the blood. However, the fact that conjoined twins with a common circulatory system sleep independently argues against a common sleep-inducing factor in the blood (Alekseyeva, 1958). But then scientists thought: maybe this putative sleep factor is not in the blood but in the brain. In order to test this idea, Legrende and Pieron (1913 ) kept dogs awake for several days. Then, they extracted cerebrospinal fluid from these animals and were able to induce sleep by injecting the fluid into the ventricular system of non-sleep deprived dogs. So that indicated the sleep inducing factor was in the brain system. Later physicians and scientists observed sleepiness and tiredness have a circadian rhythmicity which cannot be explained by the simple accumulation of a sleep-inducing factor, so the chemical theory of sleep was discarded.. MelatoninMelatonin has gained considerable attention as non-hazardous sleep-inducing pills. It is also widely used to combat the effects of jet-lag. It is a natural hormone produced by the pineal gland (top of the midbrain, between the superior colliculi). It only affects the latency to sleep and not the sleep structure. It seems to have a powerful hypnotic effect on birds but, interestingly, it seems to induce wakefulness when applied to rats during the daytime. Other substancesSeveral other substances have been suggested to have hypnogenic properties:
Adenosine builds up in the brains of animals of that are kept awake past normal times. Indeed, the material is of interest to researchers looking to improve insomnia medications. Sleep is an active processScientist have never found brain cells (neurons) that lack energy and "need" to sleep. Nor have they found neurons that run-out of neurotransmitters during the awake state and need to sleep to replenish them. There is no universal decline in firing rate during sleep. Neurons in some areas decrease their activity during sleep while neurons in other areas actually increase their firing. This is valid both for NREM as well as REM period sleep. Furthermore, this has been observed both electrophysiologically and by functional imaging studies of the human brain (see below). It's not the absence of sensory stimulation that causes sleep. The
body sensory stimulation can be severed and the animal still shows
wake-sleep cycles.
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"O Sleep, rest of all things, mildest of the gods, balm of the soul..." (Iris to Hypnos. Ovid, Metamorphoses) |