Seasonal affective disorder does not have one established cause. The strongest recognised feature is its relationship with seasonal changes, particularly reduced daylight during autumn and winter for winter-pattern SAD. Researchers believe that changes in light exposure can affect circadian rhythms, sleep regulation, hormone signalling and brain systems involved in mood. Genetic susceptibility and individual biological differences may determine why some people develop clinically significant seasonal depression while others exposed to the same daylight changes do not. SAD should therefore be understood as a disorder involving interactions between environmental seasonal signals and individual vulnerability.
Light is an important environmental signal for the circadian system. Specialised retinal photoreceptors respond to environmental light and communicate information to the suprachiasmatic nucleus, which coordinates approximately 24-hour biological rhythms. Seasonal changes in dawn and dusk alter the timing and duration of this light signal. In susceptible individuals, this may contribute to circadian misalignment, altered sleep timing and changes in alertness and mood. The mechanism is not simply that darkness directly causes sadness; rather, seasonal light changes can affect biological timing systems that interact with sleep, hormones and neural regulation.
Melatonin is one biological pathway of interest. The pineal gland produces melatonin primarily during darkness, and light suppresses melatonin production. Changes in the timing and duration of winter darkness can therefore alter melatonin patterns. In some people with winter-pattern SAD, altered circadian timing may contribute to excessive sleepiness and difficulty maintaining a stable sleep-wake schedule. However, melatonin changes are part of a larger circadian system, and measuring melatonin alone cannot diagnose SAD. The relationship between melatonin, circadian timing and depressive symptoms remains an area of ongoing research.
Serotonin and other neurotransmitter systems have also been investigated. Seasonal variation in serotonin-related brain activity has been proposed as one mechanism contributing to changes in mood, appetite and reward processing. However, depression cannot accurately be explained as a simple lack of serotonin, and SAD cannot be reduced to a single neurotransmitter abnormality. Brain circuits involved in mood, motivation and reward interact with circadian processes, and different people may have different biological pathways leading to similar seasonal symptoms.
Genetics appears to contribute to vulnerability. SAD and other depressive disorders can run in families, indicating that inherited factors may influence susceptibility. Genetic effects are likely polygenic and interact with environmental exposure rather than determining the disorder on their own. A person with a family history may therefore have increased vulnerability without necessarily developing SAD, while someone without an obvious family history can still develop it. The seasonal environment acts as an important contextual factor rather than a deterministic trigger.
Latitude and seasonal daylight patterns influence population-level prevalence. Regions farther from the equator generally experience larger seasonal changes in day length, and SAD has been studied extensively in such populations. However, prevalence is not explained by latitude alone. Cloud cover, outdoor activity, work schedules, housing, sleep patterns, socioeconomic factors and individual biology can alter actual light exposure. Geographic associations therefore support the importance of seasonal light while not proving that insufficient sunlight is the sole cause of SAD.
Sleep and daily routine can interact with seasonal vulnerability. Shorter days may encourage later waking or reduced outdoor activity, while colder or darker weather can reduce opportunities for exercise and social interaction. These behavioural changes can reinforce depressive symptoms in susceptible individuals. Nevertheless, behavioural changes are better understood as potential contributing or maintaining factors than as the complete explanation for SAD. A person can develop seasonal depression despite maintaining a regular routine, which is another reason the condition should not be reduced to lifestyle choices.
Current understanding therefore favours a multifactorial model involving seasonal light exposure, circadian regulation, sleep, neurobiology, genetics and behavioural or environmental factors. The precise causal sequence is not identical for every person and remains an active research area. The NHS identifies reduced daylight and its effects on biological rhythms as important factors while also recognising that the exact cause of SAD is not fully understood. (nhs.uk) Treatment research is consistent with this model: interventions such as appropriately timed bright light aim to modify seasonal environmental signalling, while psychological therapy and medication target other components of the depressive syndrome.
