Melatonin is a hormone produced at night by the pineal gland in the brain. In addition to its role in regulating the sleep-wake cycle, melatonin has been shown to lower blood pressure and body temperature. Joshua J. Gooley, from Harvard Medical School (Massachusetts, USA), and colleagues studied the role of exposure to electrical light between dusk and bedtime, alters melatonin production and thereby the body processes in which the hormone is involved.  The researchers evaluated 116 healthy men and women, ages 18 to 30 years, who were exposed to room light or dim light in the eight hours preceding bedtime for five consecutive days. An intravenous catheter was inserted into the forearms of study participants for continuous collection of blood plasma every 30-60 minutes for melatonin measurements. Results showed exposure to room light before bedtime shortened melatonin duration by about 90 minutes when compared to dim light exposure. Furthermore, exposure to room light during the usual hours of sleep suppressed melatonin by greater than 50%.  Reporting that: “These findings indicate that room light exerts a profound suppressive effect on melatonin levels and shortens the body's internal representation of night duration,” the team concludes that: “ Chronically exposing oneself to electrical lighting in the late evening disrupts melatonin signaling and could therefore potentially impact sleep, thermoregulation, blood pressure, and glucose homeostasis.”