Side effects - Melatonin - linked to nightmares/false awakenings/sleep paralysis?

[PingoTango]

SWIM has recently been taking 5mg melatonin supplements (bought for him by a friend in the US due to it's not being available here in the UK) to combat his bouts of insomnia - he takes one 5mg tablet each night before going to bed, at about 10:30pm. The pills do work, they help him enjoy longer periods of good quality sleep, and enable him to drop off sooner than he normally would.

He has, however, noticed that maybe once a week he will experience terrifyingly vivid nightmares, several false awakenings in a row before actually awakening, periods of paralysis combined with a ringing in the ears immediately after waking up, or (more usually) some combination of the three.

He has been taking the melatonin for maybe a month and has been experiencing these symptoms for maybe two weeks. Is there a link? Any way to negate these side effects?

Thanks in advance...