In a study by Van Mill and colleagues (2013), the authors examined the impact of sleep disturbances on the course of anxiety and depressive disorders. The sleep durations of over 1,000 individuals with depressive and/or anxiety disorders, over a period of two years, were examined. Sleep durations were characterized as short (≤ 6 hours), normal (7-9 hours) and long (≤ 10 hours).
The authors found that both short and long sleep durations were predictive of an unfavourable course of psychopathology (i.e., a chronic course), even after taking symptom severity and the participants psychotropic medications into account. Note: insomnia did not predict the course of psychopathology.
The authors (Van Mill et al., 2013) argue that short sleep durations may increase daytime tiredness, which has been linked to poorer outcomes of depression. It is also likely that those who report a short sleep duration suffer from a biologically different type of depression or anxiety that have a poorer course.
Indeed, a shared genetic background for mood disorders and short sleep has been proposed (the so-called GRI3 polymorphism). Another cause of the poor outcome could be that shorts sleepers have more waking hours, which provides an opportunity for pessimistic or depressive thoughts.
Long sleep durations, on the other hand, can be an atypical symptom of depression. The authors emphasize that several studies have indicated that people with an atypical depression have more metabolic abnormalities and inflammation than people with a non-atypical depression.
It is likely that these conditions contribute to the unfavourable course of this disorder. Also, higher levels of inflammation have been linked to longer sleep durations:
“It could well be that inflammation is an underlying common pathophysiologic mechanism leading to both increase in sleep duration and detrimental effects on course of depression/anxiety disorders.” (p. 7).
Long sleep duration has been associated with low physical activity. People who spend many hours in bed may be unable to activate themselves, resulting in increased feelings of depression, the authors argue.
The study has some limitations. Sleep durations were self-reported, and people may overestimate or underestimate their sleep durations. The authors did not use objective measures of sleep, so the results of this study rely entirely on self-reports.
Furthermore, the group size of long sleepers was only 64, which is a very small group compared to the overall sample size of over 1,000, and this group may therefore not be representative of people with both anxiety and/or depressive disorders and sleep disturbances. At last, only a two-year period (course) was examined.