Atenolol facilitates light-induced phase shifts in humans

During time-zone travel, the endogenous melatonin rhythm is often out of phase with the new local time cues. Since endogenous melatonin could act as an endogenous zeitgeber, when its secretory rhythm is out of phase it may hinder adaptation by natural zeitgebers. It is possible that by temporarily suppressing the production of melatonin, by beta-blockers for example, adaptation may be facilitated. In a double-blind, crossover study eight healthy volunteers (aged 23-30 years) took 100 mg atenolol or placebo at 1900 h on Day (D) 1. Volunteers were then exposed to bright light (approx. 1000 lux) from 0000 to 0400 h during the following night and remained in dim light (<50 lux) or darkness until 1200 h on D3. Salivary melatonin (MT) and urinary 6-sulphatoxymelatonin (aMT6s) were measured every 30-60 min and every 2 h (except when asleep), respectively. Subjective alertness and core body temperature (cBT) were also measured. aMT6s and MT were significantly suppressed under atenolol treatment on the night of D1 only. Atenolol significantly phase delayed the salivary melatonin onset by 1.8+/-0.6 h and 1.28+/-0.35 h compared with the onsets on D1 placebo leg and D2 placebo leg (i.e. onset times before and after light treatment), respectively. There were no detrimental effects on cBT or alertness. Temporary suppression of melatonin by beta-blockers may facilitate adaptation to phase shifts.