We tend to sleep less during the summer months for a similar reason sunlight has the reverse effect and prevents the gland from releasing melatonin. When it's raining, the same thing happens - the pineal gland is stimulated by the lack of sunlight, more melatonin enters the bloodstream, and we naturally feel more tired. It's the body's natural sleep aid as this is what signals to us that we're tired and it's time for bed. As research from the University of Göttingen explains, our brain's pineal gland begins to secret a hormone called melatonin when it gets dark outside.
