Sleep deprivation impairs many physiological functions, including immune regulation and metabolic control. Rats that are kept awake artificially die after two week. Upon the on-set of total deprivation, the body temperature rises, followed by a fall. Sleep-deprived rates get skin lesions. The rats "eat voraciously, but lose weight and develop malnutrition-like symptoms". Hormone levels change to mimic a state of extreme stress.
These changes are linked to eating and body weight, and as the time of deprivation continues, appetite increases, weight loss continues, and body temperature continues to fall even as the animals try to keep themselves warm. By the time they die the rats have lost a lot of weight.
Humans deprived of sleep in laboratory conditions report, even after a one night, feeling cold and hungry. It is worth noting that during the REM stage of normal sleep, the body does not thermally regulate. (More on thermoregulation.) Scientists have determined that in human sleep deprivation the decline in body temperature is 0.5° C. There is an increase in white blood cell counts and a general slowing of bodily functions.
Sleep deprivation also leads to systemic inflammation at a low level. This type of low-level inflammation is similar to that found in cardiovascular disease, diabetes, etc. and there could be a connection.
Sleep and emotion interact as most psychiatric conditions are associated with sleep disorders. There are suggestions that the even mild sleep deprivation makes emotionally healthy people cranky.
Does sleep deprivation make you crazy? No. Not in a clinical sense does sleep deprivation lead to schizophrenia or mental illness. Visual misperceptions are common among overly sleepy people, but these are not hallucinations or waking dreams, as commonly believed, and auditory hallucinations do not occur in sleep deprived people any more than in rested people. (An article in New Scientist magazine suggests that bad sleep habits can indeed cause mental illness, or something like it. If this is true, sleep problems would be a cause, in addition to a symptom of mental illness.)
Some researchers think that even short-term sleep loss causes glucose intolerance in the body and hence has the same effect on the body as a pre-diabetes state.
A study published in the Journal of Neuroscience described the reversal in sleep deprivation effects in sleep-deprived monkeys by administration of the brain chemical orexin. Scientists gave the monkeys orexin by either injection to the blood stream or through a nasal stray. The monkeys' cognitive skills improved. It is not clear whether this will help lead to a treatment for humans.
Scientists have also found that flies with extra dopamine receptors can better withstand sleep deprivation.
Evidence for the brain's need to sleep comes from work in sleep-deprived rats where scientists found sections of the brain went into a temporary sleep-like state. Sections of the cortex went a state showing brainwaves like those seen in Stage 1 and 2 sleep, "seemingly at random" according to a report. (http://www.nih.gov/researchmatters/may2011/05022011sleep.htm)
One problem with sleep deprivation experiments is that the subjects are well protected, made comfortable, and at a low stress level, while real people with sleep deprivation undergo daily lives that may contain stress. Although total sleep deprivation (no sleep) happens in extreme circumstances, much more common in day-to-day life is chronic and partial deprivation.
Migrating birds flying for weeks at a time and newborn whales (and their nursing mothers) can forgo sleep altogether without negative effects or need for catching up. While there is room for flexibility in humans, this type of suspension of the need to sleep has not been observed.