From: Annual variation in daily light exposure and circadian change of melatonin and cortisol concentrations at a northern latitude with large seasonal differences in photoperiod length
Season
Winter
Spring
Summer
Autumn
Peak level of melatonin
Mean (pg/ml)
9.44
6.48
5.97
6.13
N = 15
SD (pg/ml)
4.82
3.60
3.22
3.19