From early childhood onwards, the cycle of wakefulness and sleepiness is regulated by a circadian ‘clock’ in the suprachiasmatic nucleus (SCN) of the hypothalamus. This clock is regulated (reset) by various cues, including that of the light-dark cycle of day and night.

In the neonate, there is little entrainment of the sleep-wake cycle to the light-dark cycle (Reference Garcia, Rosen and MahowaldGarcia et al, 2001). By the age of 6 months, circadian rhythms have become established and sleep patterns have taken on a roughly 24 hour cycle of nocturnal sleep plus daytime naps. By the age of 6 years, most children sleep only at night. Total sleep time gradually decreases with age and by adolescence has stabilised to a daily average of about 8 hours and the sleep-wake cycle to slightly more than 24 hours. This cycle is reset daily by the SCN.

Insomnia is a common problem in children with sensory deficits and some learning disability syndromes. It is also a symptom of childhood psychiatric disorders such as depression and attention-deficit hyperactivity disorder. Persistent sleep disturbance in a young child can adversely affect family life. Mothers of learning-disabled children with severe sleep problems have been reported to be more irritable and less affectionate towards their children than mothers of such children without sleep problems (Reference QuineQuine, 1992). Successful treatment of the child’s sleep problems generally leads to improvements in the mother’s mental state, confidence and relationship with the child, as well as in the child’s behaviour (Reference Minde, Faucon and FaulknerMinde et al, 1994).

In adolescence, delayed sleep phase syndrome, in which the sleep-wake cycle may be particularly prolonged, is common. About 10% of otherwise normal children (Reference Smits, Nagtegaal and van Der HeijdenSmits et al, 2001) and up to 80% of children with developmental disorders (Reference Jan and O'DonnellJan & O’Donnell, 1996) are affected.

The role of melatonin

Melatonin is an endogenous hormone chemically related to serotonin. It is produced by the pineal gland in the midbrain. Body levels follow a circadian course, being stimulated by the evening onset of dim light and suppressed by bright light. The evening rise in melatonin precedes the rise in sleepiness by about 1.5-2 hours (Reference Tzischinsky and LavieTzischinsky & Lavie, 1994). The circadian rhythm of sleep consolidation (the ability to stay asleep) may also be related to melatonin levels, but this has yet to be demonstrated. Melatonin acts by resetting any disruption of SCN rhythms. Given its association with circadian rhythms, oral melatonin could be expected to treat sleep disturbances. Many case studies, case series, open and controlled studies have been published. These are summarised below.

Side-effects

Since melatonin is a natural substance, it is often assumed that its side-effect profile is benign. However, most other endogenous hormones produce considerable adverse effects in high or inappropriately phased doses and it should be anticipated that melatonin might behave similarly. Many of the published reports involved children with developmental abnormalities, which limited the scope for detection of subtle adverse effects. Most studies involved short-term use of melatonin and did not systematically inquire about side-effects.

Sheldon (Reference Sheldon1998) reported that melatonin increased seizure frequency in neurologically disabled children, an effect that disappeared when melatonin was discontinued and reappeared on further challenge. Also, Smits et al (Reference Smits, Nagtegaal and van Der Heijden2001) reported the development of mild epilepsy in one child. Whether seizure threshold is affected in the general population is unknown.

Melatonin raises levels of inflammatory cytokines such as interleukins 1 and 6 and tumour necrosis factor alpha (Reference MaestroniMaestroni, 1993), and this effect may have adverse consequences for people with inflammatory-mediated conditions such as asthma. Sutherland et al (Reference Sutherland, Ellison and Kraft2003) and Sutherland et al (Reference Sutherland, Martin and Ellison2002) reported that peak endogenous levels of melatonin were elevated in people with nocturnal asthma and were inversely correlated with measures of respiratory function; they concluded that the ‘avoidance of exogenous melatonin supplementation by persons with asthma might be warranted’.

Melatonin has contraceptive properties and can affect the onset of puberty (Reference WeaverWeaver, 1997). It also has antioxidant (Reference Pieri, Marra and MoroniPieri et al, 1994) and cytostatic (Reference BrzezinskiBrzezinski, 1997) properties. These effects are minimal at physiological levels, but may become more apparent when synthetic melatonin is taken in supraphysiological doses: the cut-off point between physiological and pharmacological levels in adults is estimated to be produced by doses of 500 μg, and this is likely to be less in children.

In clinical depression in adults, there is evidence that the risk-benefit ratio for melatonin may be adverse. Carman et al (Reference Carman, Post and Buswell1976) found that melatonin exacerbated symptoms of dysphoria, reduced sleep and led to weight loss in a small, double-blind, cross-over study in moderately to severely depressed adult patients. It has been suggested that seasonal affective disorder might respond to propranolol through the suppression of endogenous melatonin production (National Institute of Mental Health USA, 2003).

Overview

Five placebo-controlled trials in children have been identified. Most of the published literature consists of case reports and case series, which used a variety of doses and formulations of melatonin. The nature of the populations treated is diverse, and most studies report shortterm use in which the placebo effect would be highest. Oral melatonin may reduce sleep latency, but its effects on other aspects of sleep are inconsistent between studies. The optimal dosages and duration of treatment are unknown. Side-effects have been poorly evaluated to date, although worsening of seizures and asthma have been reported in the short term. Long-term side-effects are not established, although melatonin is known to have significant effects on many body systems. Melatonin is not a licensed medicine in the UK and the prescriber is fully accountable for any problems that might result from its use.