Summer is a season of increased physical activity and greater sunlight exposure, yet surprisingly many people complain of depression and anxiety. Modern medicine sometimes considers this the summer pattern of ‘seasonal affective disorder’, appearing through a mechanism opposite to the depression caused by a lack of sunlight in winter. It happens because the scorching, prolonged summer sunlight and high temperatures disrupt the brain’s delicate balance.
The very first thing affected is the secretion of melatonin, which regulates our body’s ‘biological clock’. When long daylight hours and bright nights persist, the secretion rhythm of melatonin, the sleep-inducing hormone produced by the brain, can be disrupted. Melatonin does more than just bring sleep; it is deeply involved in our mood and immunity. Irregular melatonin secretion lowers sleep quality, which in turn can lead to an imbalance in serotonin and dopamine, neurotransmitters crucial for mood regulation. Serotonin governs feelings of happiness and calm, while dopamine is involved in reward and motivation; a deficiency or imbalance in these can cause depression, lethargy, impaired concentration, and loss of appetite.
Furthermore, high summer temperatures trigger our body’s stress response. Sustained exposure to heat excessively activates the sympathetic nervous system within the autonomic nervous system, which can accelerate the heartbeat and worsen anxiety, restlessness, and insomnia. Our body continually consumes energy to maintain thermoregulation, and during this process, the brain’s energy expenditure also increases, causing brain hypofunction and fatigue. In particular, the hypothalamus oversees various biological functions including emotion, appetite, and sleep alongside thermoregulation; excessive heat stress burdens hypothalamic function and can impair overall physical and mental stability.
Dehydration is also an important factor. When fluid and electrolytes become insufficient from profuse sweating, it directly affects brain function, potentially inducing mood swings, impaired concentration, and fatigue. Even gut health is closely related. The gut microbiota is known to play an important role in brain function and emotional regulation; changes in summer dietary habits or dehydration negatively alter the gut microbiota environment, potentially worsening depressive feelings. In this way, summer depression can be understood not simply as an issue of the mind, but as a physical response arising from environmental factors acting together on the brain’s neurotransmitters, hormones, and biological clock.
