The path continues to the tiny, cone-shaped pineal gland, which secretes the hormone melatonin. It is thought that melatonin affects the regulation of behavioral changes in animals, but this has not been clearly shown in humans. Sufficiently intense light suppresses the secretion of this chemical, making it a useful marker in determining light's physical effect on behavior. The secretion of melatonin reflects light's effect on the hypothalamus, itself highly sensitive to light. This complex part of the brain regulates a multitude of body functions, playing a vital role in reproduction, thirst, hunger, satiation, temperature, emotions, and sleep patterns.
Depression is associated with disturbances in the hypothalamus.
The majority of our homes are lit by incandescent bulbs that light through heat. They lack the intensity of sunlight and produce light that is heavily inrared. It's conceivable for this purpose that they are not the safest. Widely used fluorescent lights peak in the yellow-green portion of the spectrum, wavelengths to which the eye is most sensitive. That makes them energy efficient but different from natural sunlight, notably in the blue-green spectrum where the sun's emission or radiant energy is strongest.