"The duration of melatonin secretion each day is directly proportional to the length of the night."
"The mechanism behind this pattern of secretion during the dark cycle is that activity of the rate-limiting enzyme in melatonin synthesis - serotonin N-acetyltransferase (NAT) - is low during daylight and peaks during the dark phase."
"the pineal.. transduces signals from the sympathetic nervous system into a hormonal signal."
http://www.vivo.colostate.edu/hbooks/pathphys/endocrine/otherendo/pineal.html