It’s no accident that most people tend to sleep at night and are awake during the day. Our sleep-wake cycle is determined by our circadian rhythm, the body’s internal clock. Like old-time clocks, this internal clock needs to be reset every day, and is adjusted by first exposure to light in the morning.
How does circadian rhythm work?
Our circadian rhythms are controlled by multiple genes and are responsible for a variety of important functions, including daily fluctuations in wakefulness, body temperature, metabolism, digestion, and hunger. Circadian rhythm also controls memory consolidation (the formation of long-term memories occurs during sleep); the timing of hormone secretion (for example, the hormones controlling body growth work mostly at night); and body healing.
While the circadian sleep phase typically occurs at night, there are a range of times during which the sleep phase can occur, with some people programmed to sleep from early evening to early morning (known as morning larks), while others stay up late and sleep late (known as night owls). In addition to determining the timing of their sleep, a person’s circadian tendency can also affect their choice of emotional coping skills, such as assertiveness or rationalization, and their predisposition to psychological disorders.
How does your circadian rhythm impact your mood?
An irregular circadian rhythm can have a negative effect on a person’s ability to sleep and function properly, and can result in a number of health problems, including mood disorders such as depression, anxiety, bipolar disorder, and seasonal affective disorder.
A recent study suggested that the night-owl type might have a greater predisposition to psychological disturbances. The authors found that the different circadian types were likely to have different coping styles to emotional stressors, and the ones adopted by the morning larks seemed to result in better outcomes and fewer psychological problems. This was a correlational study, so the reason for adopting different styles wasn’t explained, but this study emphasizes the great impact circadian rhythms have on health and functioning.
Depression and circadian rhythm
Most of the evidence on the relationship between mood problems and circadian rhythm comes from studies of shift workers, whose sleep periods are out of sync with their circadian rhythm. Multiple studies show an increased prevalence of depression in night-shift workers. One meta-analysis showed that night-shift workers are 40% more likely to develop depression than daytime workers. Conversely, circadian rhythm disturbances are common in people with depression, who often have changes in the pattern of their sleep, their hormone rhythms, and body temperature rhythms.
Symptoms of depression may also have a circadian rhythm, as some people experience more severe symptoms in the morning. The severity of a person’s depression correlates with the degree of misalignment of the circadian and sleep cycles.
Many successful treatments of depression, including bright light therapy, wake therapy, and interpersonal and social rhythm therapy, also directly affect circadian rhythms. (For the impact of circadian rhythm on the occurrence and treatment of depression related to bipolar disorder,