Melatonin is a hormone that has many functions which is produced at night by the pineal gland, it regulates the sleep-wake cycle, body temperature, blood pressure, and glucose tolerance; lack of melatonin can create serious medical problems.
A study conducted by Harvard Medical School looked at the effects of electric lighting between dusk and bedtime on the suppression of melatonin. Subjects were 116 healthy volunteers, aged 18-30 years, who were exposed to dim lighting or to room lighting for 8 hours preceding bed time on 5 consecutive days. Periodic blood draws measured melatonin production for each subject during the 8 hours. The results showed that those exposed to room lighting before bedtime had shortened melatonin production by 90 minutes compared to those who were in dim light. Additionally, when exposed to room light during normal sleep hours, the sleep suppressed melatonin was reduced by 50 percent.