According to the World Association of Sleep Medicine statistics, sleep issues represent a global epidemic affecting up to 45% of the world’s population.
Sleep is a basic necessity for the human body affecting the physical, mental and emotional states alike. There have been several studies demonstrating an association between sleep duration and morbidity and mortality.
Sleep disorders are common and affect sleep quality and quantity, leading to increased morbidity. Patients with sleep disorders can be categorized as those who cannot sleep, those who will not sleep, those with excessive daytime drowsiness, and those with increased movements during sleep. Insomnia, defined as difficulty initiating or maintaining sleep that results in daytime impairment, is diagnosed using history findings and treated with cognitive behaviour therapy, with or without sleep hypnotics (1).
Research over the past decade has documented that sleep has a critical role in promoting health, with sleep disturbance posing a potent risk to infectious disease, the incidence of depression, and to the occurrence and progression of cardiovascular disease and even cancer (2). It is important to recognize the role of sleep and circadian disruption in the development, progression and morbidity of metabolic disease (3).
Sleep helps the brain commit new information to memory through a process called memory consolidation. In studies, people who had slept after learning a task did better on tests later. Active consolidation of memories is established during sleep, originating from the reactivation of newly encoded memory representations (4).
The sleep/wake cycle is accompanied by changes in the circulating numbers of immune cells. The study of different blood cell populations demonstrate that acute sleep deprivation directly affects granulocyte levels and diurnal rhythmicity in a manner similar to the body's immediate immune response when exposed to stress (5). This implies that proper and adequate sleep is essential for good health, enabling the immune system to defend against infections and illness.
There is increasing interest in the possibility that disruption of normal circadian rhythm may increase the risk of developing cancer. Those who work night shifts may show altered night time melatonin levels and reproductive hormone profiles that could increase the risk of hormone-related diseases such as breast cancer. Epidemiologic studies are now beginning to emerge suggesting that women who work at night, and who experience sleep deprivation, circadian disruption and exposure to light-at-night are at an increased risk of breast cancer and possibly colorectal cancer (6).
Sleep deprivation can trigger weight gain, as the two vital physiological responses are linked by hormonal paths and disrupting one can affect the other (7).
Leptin is also known as the “satiety hormone” and inhibits feelings of hunger, instead making us feel full. When fat is stored and reaches a certain level, circulating levels of leptin are increased. Conversely the “hunger hormone” ghrelin is an appetite stimulating peptide that is released within the gastrointestinal tract when the stomach is empty, and then reduced again when the stomach is stretched and full (8). Leptin and ghrelin help regulate body weight and food intake and overstimulation or understimulation at the extreme, can result in obesity, anorexia, or cachexia (wasting syndrome) (9).
A shorter sleep duration causes hormonal changes that contribute to hyperphagia (excessive hunger), insulin resistance and obesity. In a study with 769 participants, results provided evidence that sleep duration is inversely associated with serum leptin, the “satiety hormone” (10). In a study with over 1000 participants, it was found that those with only short amounts of sleep had reduced leptin and elevated “hunger hormone” ghrelin. These differences in leptin and ghrelin are likely to cause an overall increased appetite, possibly explaining the increased BMI (body mass index) observed with the short sleep duration group (11).
In a study of just under 1000 young men, those with a high level of sleep disturbance had a 29% lower adjusted sperm concentration, illustrating associations between sleep disturbances and semen quality (12).
Recent studies suggest that shift workers who experience exposure to light at night could be at increased risk for adverse reproductive outcomes.
The reproductive system is based on cyclical patterns of circulating hormones. It is susceptible to shifts in circadian rhythms which can be brought about by sleep disturbances, altered melatonin production or exposure to light at night amongst others (13). Millions of people around the world work a night shift, including health care workers, law enforcement, firefighters and manufacturing workers.
Long-term epidemiologic studies on large numbers of night and rotating shift workers have suggested an increase in the incidence of breast and colon cancer in these populations, though more specific studies are required (14).
Shift workers who experience sleep disturbances and exposure to light at night could be at increased risk for alterations in physiologic functions that are circadian in nature with possible implications for fertility and other cycle-related aspects of women's health (15).
Any impairment on menstrual physiology can impact quality of life, including mood changes, body image, infertility and pregnancy complications. There is evidence of a relationship between light exposure and the subsequent influenced melatonin secretion, and irregular menstrual cycles and disordered ovarian function (16).
A study revealed data indicating that rotating shift work can increase the risk of menstrual cycle irregularity (17).
Another study included nurses in America and Italy, with findings that the rotating shift workers were possibly associated with higher rates of shorter cycles and inadequate luteal phases compared to those working fixed shifts. Menstrual function may be affected by stressful work conditions (18).
Disruption of circadian rhythms is associated with disturbances in menstrual function. Female shift workers compared to non-shift workers are more likely to experience and report menstrual irregularity. There is also accumulating evidence that circadian disruption may increase the risk of breast cancer in women, possibly due to altered light exposure and reduced melatonin secretion (19).
Sleep loss may result in irritability, impatience, inability to concentrate, and moodiness. There is a strong correlation between sleep and depression with almost 75% of depressed patients suffering from insomnia symptoms (20). Such symptoms adversely impact on quality of life, causing great distress and can be a strong risk factor for suicide. Epidemiological studies have revealed that the relationship between sleep disturbance and depression is so strong that insomnia in non-depressed individuals represents a risk factor for later development of depression (20).
Sleep plays an important role in physical health because sleep is involved in the healing and repair of heart and blood vessels. Ongoing sleep inadequacy is linked to an increased risk of heart disease, kidney disease, high blood pressure, diabetes, and stroke (21).
Lack of sleep affects circulating thyroid hormone levels and the thyroid is a major regulator of metabolic rate, modulating oxygen consumption, heart contractility and cardiac output, and affects all aspects of carbohydrate metabolism (22).
The body repairs and regenerates tissues during the deep sleep phase, can build bone and muscle, and appears to strengthen the immune system. Unfortunately with age, sleep becomes increasingly erratic and the deep sleep phase less frequent. Aging is associated with shorter time spans of sleep, although studies show the amount of sleep needed doesn't diminish with age.