Light suppresses melatonin while darkness stimulates its synthesis. Many people have trouble falling asleep. Delayed sleep phase syndrome results in late sleep onset, despite normal sleep architecture and total sleep duration. Melatonin has been shown to improve sleep latency (the time it takes to fall asleep) in several randomized controlled studies. Rather than immediately prior to sleeping, melatonin works best when given two hours before sleeping. Melatonin is also useful for jet lag, irregular sleep-wake rhythms, and shift work sleep disorder.  Exogenously administered melatonin has phase shifting properties, and the effect follows a phase- response curve (PRC) that is about 12 h out of phase with the PRC [phase response curve] of light Melatonin administered in the afternoon or early evening will phase advance the circadian rhythm, whereas melatonin administered in the morning will phase delay the circadian rhythm (Fig. 2). The magnitude of phase shifts is time-dependent, and the maximal phase shifts result when melatonin is scheduled around dusk or dawn. The effect of exogenous melatonin is minimal when administered during the night, at least during the first-half of the night.

additional information

Circadian rhythm sleep disorders are common in clinical practice. The disorders covered in this review are delayed sleep phase disorder, advanced sleep phase disorder, free-running, irregular sleep-wake rhythm, jet lag disorder and shift work disorder. Bright light treatment and exogenous melatonin administration are considered to be the treatments of choice for these circadian rhythm sleep disorders. Circadian phase needs to be estimated in order to time the treatments appropriately. Inappropriately timed bright light and melatonin will likely worsen the condition. Measurements of core body temperature or endogenous melatonin rhythms will objectively assess circadian phase; however, such measurements are seldom or never used in a busy clinical practice. This review will focus on how to estimate circadian phase based on a careful patient history. Based on such estimations of circadian phase, we will recommend appropriate timing of bright light and/or melatonin in the different circadian rhythm sleep disorders. We hope this practical approach and simple recommendations will stimulate clinicians to treat patients with circadian rhythm sleep disorders.