Melatonin production - requires nighttime, or just full darkness?
The immediate thought came to my mind was relation between Melatonin and Serotonin. Because both hormones have sleep related function. I read that both are light sensitive. Melatonin is produced by pineal gland in brain whereas Serotonin is at gut and CNS.

Light dependence(Wikipedia) of Melatonin is explained here where they say blue light produces more Melatonin and wavelength of light &lt;530nm will have effect on Melatonin production. If the light intensity is more then Melatonin will not be produced. So what they say is if there is darkness(artificial) or dim light environment then this hormone will be produced and induces sleep. Almost everywhere they explain Melatonin whose production is sensitive to light.

(But even in the afternoon when there is bright light some of us feel sleepy which is mainly due to Serotonin released after heavy meal which is produced from tryptophan. This will again effect the sleep. It is obvious that without food we can not sleep at night. So Serotonin and Melatonin are interrelated as explained here Link1 and Link2).