Summary Treatment of circadian rhythm sleep disorders (CRSD) may include light therapy, chronotherapy and melatonin. Exogenous melatonin is increasingly being used in patients with insomnia or CRSD. Although pharmacopoeias and the European food safety authority (EFSA) recommend administering melatonin 1–2 h before desired bedtime, several studies have shown that melatonin is not always effective if administered according to that recommendation. Crucial for optimal treatment of CRSD, melatonin and other treatments should be administered at a time related to individual circadian timing (typically assessed using the dim light melatonin onset (DLMO)). If not administered according to the individual patient's circadian timing, melatonin and other treatments may not only be ineffective, they may even result in contrary effects. Endogenous melatonin levels can be measured reliably in saliva collected at the patient's home. A clinically reliably DLMO can be calculated using a fixed threshold. Diary and polysomnographic sleep-onset time do not reliably predict DLMO or circadian timing in patients with CRSD. Knowing the patient's individual circadian timing by assessing DLMO can improve diagnosis and treatment of CRSD with melatonin as well as other therapies such as light or chronotherapy, and optimizing treatment timing will shorten the time required to achieve results.