As days become shorter susceptible people typically develop a cluster of symptoms associated with seasonal affective disorder (SAD): decreased energy levels, difficulty waking up, increased sleeping, weight gain, social withdrawal, difficulty concentrating and depressed mood. These symptoms usually remit in the spring with the increase in photoperiod. There is also evidence for seasonality (i.e. less severe changes in mood than observed in SAD) in the general population. SAD is attributed to the bodys response to decreased daylight and prevalence correlates positively with increasing latitude. Although there is no consensus regarding the pathophysiological mechanisms underlying SAD, proposed mechanisms include circadian phase shift and retinal subsensitivity to light. Cortisol secretion has a well-established circadian rhythm which is synchronised with the light/dark and sleep/wake cycles. This rhythm is characterised by a diurnal decline, gradually increasing concentrations during late sleep and a marked burst of secretion following awakening: the cortisol awakening response or CAR. The CAR, unlike the rest of the diurnal cycle of cortisol is sensitive to light: enhanced by awakening in light and by dawn simulation. It was hypothesised that the CAR may be attenuated in winter in those with seasonality and SAD. In the first study a non-clinical population with a greater propensity for self-reported seasonality presented with a smaller than average CAR in winter. In a separate study it was shown that in summer, SAD and control participants had similar psychological and cortisol profiles, including the CAR. However in winter, the CAR was significantly attenuated in SAD participants in comparison to controls and themselves during summer. Furthermore in winter, general dysphoria correlated inversely with the CAR, indicating that participants reporting greater negative affect and lower arousal exhibited lower CARs. This paper will review the role of light in the regulation of the CAR and associations with SAD and seasonality.
Declaration of interest: There is no conflict of interest that could be perceived as prejudicing the impartiality of the research reported.
Funding: Declaration of Funding: This work was supported by the Bial Foundation.