Sleep is NOT a bank. You can't store it up and then go days on little sleep. Nor can you overdraw without penalty and then make up sleep needs on weekends.
Sleep deprivation reduces emotional intelligence and constructive thinking skills. It affects body weight, the immune system, and more.
Total sleep deprivation can cause higher energy usage (increased metabolism and consequent weight loss), reduction in higher-level cognitive abilities, and a reduction in the effectiveness of the immune system and increased susceptibility to infection. In bad cases, people with total sleep deprivation can get skin lesions and irregular heartbeats.
Partial sleep deprivation is harder to pin down, partly because there is no "right" amount of sleep for a person and because the desire for sleep and tendency to fall asleep fall along a continuum. However, we can say that partial sleep deprivation has some of the same effects, although at lower levels. Chronic short sleep, arguably a form of partial sleep deprivation, leads to increased body mass (in contrast to short-term total sleep deprivation which makes the subject lose weight.
There are no viable biomarkers for sleep debt, so any formal diagnosis has to be behavioral.
A person who loses one night’s sleep will generally be irritable and clumsy during the next day and will either become tired easily or speed up because of adrenalin. After missing two night’s sleep, a person will have problems concentrating and will begin to make mistakes on normal tasks. Three missed nights and a person will start to hallucinate and lose grasp of reality.
Sleep deprivation seems to increase levels of inflammatory mediators such as IL-1, IL-6 and TNF. What does this mean? It doesn’t definitely mean anything, but it might mean a lot of things. Inflammation is associated with (scientists aren’t willing to use the word "cause") a number of long-term chronic illnesses such as heart problems and cancer. There is no smoking gun leaking restricted sleep to these conditions, but it is reasonable to assume poor sleep increases the chance of developing health problems.
Sleep debt is associated with melancholic symptoms and clinical depression.
It may increase the severity of the chronic problems seen in conjunction with aging. It affects carbohydrate metabolism the same way diabetes does (at least in the short run) and produces higher levels of the stress hormone cortisol in the evenings and generally more activity of the sympathetic nervous system.
People with sleep debt also take more risks. A test of sleep deprived blackjack players found they correctly assessed the increased risk of certain bets in the same manner as rested players did, but were more willing to place bets on the higher risk propositions.
The decline in performance can be measured through behavioral tests and advanced imaging systems can even show lower levels of bloodborne oxygen being delivered to important areas in the brains of those deprived of sleep for 24 hours.
Scientists found that a single night of sleep loss (total deprivation) reduces a person’s ability to distinguish between relevant and irrelevant stimuli in the visual working memory, while 4 consecutive nights of sleep limited to 4 hours/night does not reduce this ability nearly as much. This difference has implications for military commanders assigning personnel in stressful situations.
The cognitive penalty affects not only math and language skills, but also emotional intelligence and soft skills. Modern understanding of intelligence holds that each individual has multiple types of intelligence – e.g. interpersonal, coping, analytical. Tests have shown that all types of intelligence fall after sleep deprivation. The tendency to magical thinking and belief in superstition rises.
Sleep deprivation basically makes you dumber.
Sleep deprivation reduces vigilance - the capacity for sustained attention. It also reduces the size and speed of working memory, and some interesting facts about this effect on short-term memory has been uncovered. Some psychologists break working memory down into two parts: visual working memory and verbal working memory. Verbal memory encompasses ability to recall facts the subject hears or reads. It declines when a person has not had sufficient sleep.
Visual working memory is limited even in the most well rested brains. Experts estimate it can hold only a few objects. The well-rested brain can move items in and out of the visual memory and this ability is an important part of overall working memory and has a great impact on tests of overall intelligence. Identifying and deciding what objects to put in the visual working memory is an important part of higher cognitive function.
Experiments have shown a difference between partial sleep deprivation and total sleep deprivation when it comes to the effect on visual working memory. The capacity of this type of memory is essentially unchanged when a person is sleep deprived. The capacity is about the same as it is in a rested person. The ability to move items into and out of visual working memory, however, is degraded substantially in people with total sleep deprivation, but not much in those with partial sleep deprivation.
This filtering affects the effectiveness of the visual working memory. People with partial sleep deprivation therefore suffer a decline in visual working memory’s contribution to overall cognitive ability, but not as severe a decline as those with total sleep deprivation. Total deprivation reduces the ability to ignore irrelevant stimuli and therefore causes the mind to be more cloudy.
Although insufficient sleep is a risk factor for obesity, total sleep deprivation causes weight loss. Animals that are forced to stay awake lose weight rapidly, and even humans report feeling cold when forced to remain awake for extended periods (humans are not subject to extreme sleep deprivation tests the way animals are, but anecdotal evidence from people who accidentally face total sleep deprivation indicates weight loss.)
How can we resolve this apparent contradiction? Does sleep loss make us lose weight or gain weight? Scientific investigation has shown the body uses more energy during periods of sleep loss, but that the rate of energy use depends on the rate of sleep loss. During total sleep deprivation, the brain accumulates sleep debt at a rapid rate, and energy use is high – so high that the animal or person loses weight.
In the more common situation of partial sleep deprivation – chronic insufficient sleep – the rate of extra energy expenditure is low and the body can accommodate. Here the effects of the sleep loss on the appetite system swamp any extra energy consumed.