Why does the sun make you tired?
Exposure to sunlight results in a number of chemical processes in the body, including absorption of vitamin D, vitamin D3 sulfate and cholesterol sulfate. In addition, exposure to heat results in the body's cooling system kicking in, which causes sweating. These natural processes cause the body to work harder, even at rest, which may result in a feeling of sleepiness.
Sources: blog.zocdoc.com, webmd.com, livinglibations.com