Melatonin is usually produced in darkness unlike Serotonin. But it plays an important role in our circadian rhythm and sleep. That is a proposed reason why we could feel tired, sad, or groggy during rainy days since it is usually dark outside and our body produces more melatonin thinking there is no light which would have triggered Serotonin.

Importance of Melatonin

Serotonin plays a big role on regulation mood swings, emotions, and appetite. With lack or low Serotonin levels, it plays a role on depression, anxiety, mania, and other physical and mental health conditions. 

Michigan State University

When Sunlight enters your eyes, it stimulates parts of the retina which then cues the brain to produce Serotonin. The more the brightness, the more it gets produced. 

Impact of sunlight in part of Serotonin release.

Lux is the measure for the sunlight intensity perceived by the human eye. A sunny day provides more than 10,000 lux, a cloudy day provides little more tha 1000 lux, and being indoors provides us with only 50-100 lux. With research 500-1000 lux is required for our body to get the biological signal. 

Intensity of Sunlight for impacting Serotonin Levels

Meesters, A. N., Schroevers, M. J., Stewart, R. E., Fleer, J., & Meesters, Y. (2022). Seasonal patterns in mindfulness in people with seasonal affective disorder (SAD). Journal of Affective Disorders Reports, 8, 100341. https://doi.org/10.1016/j.jadr.2022.100341

Seasonal patterns in mindfulness in people with seasonal affective disorder (SAD)

Importance of Serotonin

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.

Light and Sunlight gives Happiness.

An Australian study that measured levels of brain chemicals flowing directly out of the brain found that people had higher serotonin levels on bright sunny days than on cloudy ones. That effect remained no matter how cold or hot the weather was. The rate of production of serotonin by the brain was directly related to the prevailing duration of bright sunlight and rose rapidly with increased luminosity. The findings are further evidence for the notion that changes in release of serotonin by the brain underlie mood seasonality and seasonal affective disorder.

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