"Our results show that there is a substantial effect of genetics factors in sleep terrors," though no specific genes behind the phenomenon have been identified, Dr. Bich Hong Nguyen of the Sleep Disorders Center at Montreal's Sacre-Coeur Hospital and colleagues reported.

In the study of 390 sets of twins, which appeared in the journal Pediatrics, the researchers found identical twins were much more likely to both experience night terrors than fraternal twins.

Identical or monozygotic twins have nearly identical genetic makeups, while fraternal or dizygotic twins do not. Twins are often studied because their similar genetic makeup can provide information about diseases and other issues.

The researchers said that environmental factors could be part of the cause of night terrors since the twins were being raised together in similar settings.

Overall, they found that 37 per cent of the twin sets had sleep terrors at 18 months, with the problem disappearing a year later for about half of them.

Earlier studies cited in the report have shown genetic factors likely are a factor in some cases of sleepwalking, sleep talking and night terrors. Another study said 19 per cent of 4- to 9-year olds experienced night terrors.

In contrast to nightmares, "the onset of sleep terrors is abrupt and frightening, usually sudden arousal with screaming," the study said. "During these events children seem confused and disoriented. Any attempt to awaken them may increase their agitation and prolong their episode."

However night terrors are brief, cease abruptly and the child usually returns to a deep sleep remembering nothing about the episode, the study said.