Volunteer students from 18-22 years with more than one year of GSM mobile phone usage were included in the study and divided into two groups (n=30 each): 1) subjects which used their mobile phones for more than 2 hours/day and 2) subjects which used their mobile phones for 2 hours/day or less.
From each subject, 3 blood samples were taken on the same day: one sample in the morning (3-4 am), one at noon (1-2 pm) and one in the evening (5-6 pm).
No significant differences were found in the melatonin levels over the day between both groups.
There was a non-significant negative correlation (r=-0.66) between hours of mobile phone usage and levels of melatonin.
(remark EMF-Portal: the results were not adjusted for confounders (e.g. sleep parameters, see Saxena et al. 2014).)
The authors conclude, that there might be some evidence for a relationship between mobile phone usage and the circadian rhythm of the melatonin concentration in the blood of students.