Central fatigue is a form of fatigue originating in the central nervous system that is not necessarily a response to physically demanding conditions, but other factors, like levels of certain compounds in the body. In central fatigue, people feel a sense of intense tiredness and may want to lie down or sleep. There are a number of theories to explain how this type of fatigue develops, and it is a topic of interest for researchers, as it has been implicated in conditions like chronic fatigue syndrome.
Very high levels of serotonin are observed in people with central fatigue, and many people also have elevated levels of tryptophan, a precursor to serotonin. Some researchers have suggested that this state occurs when too much tryptophan passes over the blood-brain barrier in response to metabolic changes, triggering serotonin overproduction. People have also explored the role of glycogen stores, with some studies indicating that the central nervous system responds to depleted glycogen by triggering central fatigue to protect the body from damage.
This state appears to be tied in with circadian rhythms. During normal sleep points in the sleep-wake cycle, organisms often develop central fatigue even if there is no physical reason to be tired. It could be thought of as the central nervous system's way of flashing the lights to warn people that the show is about to begin. People with disrupted sleep-wake cycles can develop extreme fatigue, sometimes at odd hours. This can be seen in people with insomnia, who may experience central fatigue during the day but still be unable to sleep.
One theory about central fatigue suggests that it may kick in when people overexercise. If people exercise too long, causing core temperature to rise while levels of stored glycogen drop, the body is at risk of organ damage. The central nervous system may trigger a fatigued state to get the athlete to stop and rest with the goal of making the person more aware of symptoms of low blood glucose and overheating. Once people have rested, had a snack, rehydrated, and cooled down, they may have energy again.
People with conditions associated with central fatigue sometimes have difficulty explaining it, because it is not directly connected to physical activity levels. They may be able to engage in regular physical activities one day and be too tired to move the next, due to disturbances in the chemical balance of the brain.