Sleep debt  Sleep debt or sleep deficit is the cumulative effect of not getting enough sleep. A large sleep debt may lead to mental or physical fatigue. There are two kinds of sleep debt: the results of partial sleep deprivation and total sleep deprivation. Partial sleep deprivation occurs when a person or a lab animal sleeps too little for several days or weeks. Total sleep deprivation means being kept awake for at least 24 hours. There is debate in the scientific community over the specifics of sleep debt, and it is not considered to be a disorder. It has been found that there are 80 proteins in the brain, called "sleep need index phosphoproteins" (SNIPPs), which become more and more phosphorylated during waking hours, and are dephosphorylated during sleep. The phosphorylation is aided by a protein called Sik3. There is a kind of mutant mouse called Sleepy with a mutated version of this protein (called SLEEPY) which is more active than the normal version, resulting in the mice being constantly sleepy and showing more slow-wave activity during non-REM sleep, which is the best measurable index of sleep need known. Inhibiting Sik3 decreases phosphorylation and slow-wave activity in both normal and mutant mice. Chronic sleep debt has a substantial health impact on the human body, specifically on metabolic and endocrine functions. A study published in "The Lancet" investigated the physiological effects of sleep debt by assessing the sympathovagal balance (an indicator of the sympathetic nervous system activity), thyrotropic function, HPA axis activity as well as the carbohydrate metabolism of 11 young adult males whose sleep period for six nights was either restricted to four hours per night or extended to 12 hours in bed per night. Results revealed that in the sleep-debt condition, thyrotropin concentrations were decreased while lowered glucose and insulin responses indicated a clear impairment of carbohydrate tolerance, a 30% decrease than in the well-rested sleep condition. On the other hand, males who were sleep-restricted showed significantly elevated concentrations of evening cortisol (the "stress" hormone) and elevated sympathetic nervous system activity in comparison to those who enjoyed a full sleep, over a period of 6 nights. Chronic sleep debt has a detrimental impact on human (neuro)physiological functioning and can disrupt immune, endocrine and metabolic function while increasing the severity of cardiovascular and age-related illnesses over a period of time. Accumulated and continuous short-term sleep deficit has been shown to increase and intensify psychophsyiological reactions in humans to emotional stimuli.