A drug free way to stay happy is increasing exposure to sunlight. Our Circadian Rhythm includes expression of Serotonin during the day and Melatonin at night. When we wake up in the morning to sunlight, this signals our body that it is time to produce Serotonin since we start becoming awake and alert. Waking up to a rainy day now proposes the body that light has not come yet. Thereby Melatonin.