"Health is a state of complete physical, mental, and social well-being and not merely the absense of disease or infirmity." - World Health Organization Constitution
During sleep your body gets a rest. In Non-REM sleep the pulse and blood pressure drop, easing the strain on your cardiovascular system. In deep stage 3 sleep this relaxation is even more pronounced. This regular dip in strain is probably important for long term heart health, and sleep disorders that disrupt NREM sleep are linked to increased risk of stroke.
Insufficient sleep causes the body's immune and stress response systems to act as if there is a threat. Adrenaline, cortisol, and other stress hormones more common during the day are released even at night, People with constant lack of sleep often have higher levels of C-reactive protein which is a sign of inflammation and may be a risk factor for heart disease. Sleep disorders contribute to the risk of a host of maladies, including cancer, high blood pressure, diabetes, stroke, and heart disease. They change (mostly for the negative) how mentally agile we are and how we learn and cope with the world.
Sleep is also a time when growth hormone levels increase, tissue is renewed (there really is a beauty sleep), and cytokine levels rise. Cytokines are essential to the immune system, and the old idea that sleep helps you fight off illness has scientific basis. Sufficient sleep also reduces your chance of obesity. Sleep and lack thereof is closely tied up with mental health. Insomnia is a symptom of most mental illnesses, and while bad sleep habits may not cause declines in mental health all the time, they cannot help.
Daytime mood and performance of mental and physical tasks are affected by the previous night's sleep. Some medical disorders get worse at night: asthma and acid reflux are among them.
Sleep disorders – the preferred term in sleep medicine and here at Sleepdex, are not always "diseases." How do we know if someone has a disease? We say they do by
1) defining disease and seeing if conditions fit it
2) accepting diagnosis of doctor
This method does not capture the many cases with no symptoms.
Definitions can change over time (e.g. diabetes: blood glucose levels targets were revised downward upon discovery of complications at low levels = now more people have it.) As we learn more about sleep the already-fuzzy borders of disorders may move.
Insomnia is associated with many medical and psychiatric disorders, although whether it is a symptom or causes the disorder is often unknown. Common ones include dyspnea and pain. Scientists have estimated that 40% of those with chronic insomnia have a psychiatric disorder. In the past, when a patient had both insomnia and psychiatric problems, the mental illness was treated with the assumption that it cleared up, so would the insomnia. When non-sedating psychiatric drugs were developed it was found that insomnia was “refractory” to the alleviation of depression.