Melatonin: nocturnal portion (02:00 am - 03:00 am) in saliva

Melatonin is a hormone synthesized by the pineal gland, it controls circadian rhythms: sleep and wake. Maximum hormone is produced between 01:00 and 03:00 a.m. during sleep in complete darkness (70% of the daily melatonin production). Even brief light turned on at night can significantly reduce the hormone synthesis. Melatonin inhibits the secretion of anterior pituitary gland hormones: FSH, LH, ACTH, TSH, STH. Melatonin has antioxidant properties, antineoplastic action and oncostatic activity.
Melatonin production is violated with jetlag - time zone changing syndrome (rhythm of hormone secretion is shifted); insomnia due to shift work; sleep phase delay syndrome; depressive disorders, etc.
Melatonin synthesis decreases distinctly with age. Melatonin is segregated from tryptophan through serotonin formation, that is converted into melatonin under N-acetyltransferase and hydroxyindole-O-methyltransferase influence.
Predominant part of melatonin is metabolized in the liver with P450 cytochrome, turning into 6-hydroxymelatonin, conjugated with sulfate to form melatonin sulfate after biological function.
The decrease in melatonin levels is clinically significant, it is observed when:

time of falling asleep and / or waking up is increased;
sleep quality is poor;
insomnia;
Alzheimer's disease;
immune system is weakened;
taking sedatives