THREE young brothers from a village near Quetta show normal physical activity during the daytime after sunrise but become paralysed after sunset, said Dr Tahir Anis in these columns (May 9) and suggested this condition to be the ‘familial periodic paralysis’.

The familial periodic paralysis is a group of genetically inherited disorders in which transient paralysis can occur due to either raised or lowered level of potassium in blood, and also by the raised level of thyroid hormone; but unlike the symptoms of solar kids, it does not show any rhythmic pattern.

The solar kids’ symptoms show a typical circadian rhythm. The circadian clock or biological clock is present in every living being, including the humans. It determines the daily rhythmic functions, particularly the cycle of sleep and wakefulness. The circadian rhythms continue to operate even if the individual is not exposed to direct sunlight.

Various disorders of circadian rhythm in humans are reported. One of these is the advanced sleep phase syndrome (ASPS) or advanced sleep phase disorder (ASPD), in which the affected person falls asleep several hours earlier (at about sunset) than the usual bedtime of other people, and rises very early in the morning. The condition of these children fits into this description.

Their paralytic condition may be due to another disorder of the sleep cycle in which the rapid eye movement (REM) phase abruptly starts without being preceded by the non-REM phase. Normally the REM starts when a person has been asleep for about 100 minutes; most of the dreams are said to occur in this phase. One typical feature of REM is the paralysis of muscles. If awakened in this phase, one cannot move or speak. Everyone undergoes this paralysis during their sleep, mostly unknowingly.

However, in an abnormally early onset of REM, the paralysis occurs even before the brain is asleep and the person can feel this. The ASPS or ASPD is known at least from the late 1970s.

In 1999, Dr C.R. Jones and others recognised its hereditary status as an autosomal dominant trait, thus renamed it as the Familial Advanced Sleep Phase Syndrome (FASPS). Then in 2001 Dr Ying-Hui Fu and her research group identified a mutation in the gene ‘hPer2’ as the cause of FASPS. This paper has 1,050 citations to date and is regarded as the most influential work on the genetic basis of circadian rhythms.

Dr Fu is a professor of neurology in the University of California, San Francisco. Her lab’s chief discoveries include the identification of causative genes and mutations for circadian rhythm sleep schedule in humans.

She could be contacted for her advice on the solar kids’ case.

Prof (Dr) Surayya Khatoon

University of Karachi

Published in Dawn, May 18th, 2016