Sleep disturbance, consisting of insomnia (a subjective perception of inadequate sleep) and sleep deprivation (an objectively measured decrement in sleep), is associated with both manic and depressive episodes [54]. For example, Harvey and colleagues found that 69–99% of patients experienced decreased need for sleep during manic episodes and 23–78% reported hypersomnia when depressed [54]. Rates of insomnia during mood episodes vary greatly with up to 97% of patients experiencing difficulties falling asleep or staying asleep during manic episodes [54]. These data reflect the prevalence of comorbid sleep disturbance amongst individuals with MDD or bipolar disorder. One potential explanation for the high degree of sleep disruption amongst individuals with MDD or bipolar disorder and its social impact is the social zeitgeber theory. This theory states that stress causes disruptions in social, daily routines (e.g., meals, exercise, sleep schedule) which disrupt circadian biological rhythms (e.g., body temperature, hormones). These disruptions may trigger episodes of mania or depression [55]. Studies have found that exercise is a key zeitgeber, or time cue, that helps to entrain other social rhythms [56, 57]. Thus, exercise could
