“How to increase serotonin in the human brain without drugs”Simon N. Young
Exposure to bright light increases serotonin. Postmortem brain studies showed that serotonin levels are higher in those who died in summer than in those who died in winter.
Fatigue during exercise is associated with elevated brain and plasma tryptophan and serotonin synthesis and a decrease in the plasma level of the branched chain amino acids (BCAAs) leucine, isoleucine and valine. The BCAAs inhibit tryptophan transport into the brain. Tryptophan is an effective mild hypnotic, a fact that stimulated the hypothesis that it may be involved in fatigue and recovery after exercise (many reject this last idea).
SO EXERCISE IN THE SUN!