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It's 3 a.m. You've woken from a dead sleep because you've heard a thud in the darkness or a floorboard creaking downstairs. Or perhaps you've woken because you have the unmistakable sensation you're not alone.

During daylight, these phenomena might be dismissed as the heating, your neighbors or the sounds of an old house settling. But in the middle of the night they can feel a tad more sinister.

Your mind might immediately start reeling, jumping to the worst-case scenario—wondering if you've been broken into, or if you're being haunted by some supernatural force. In reality, the explanation is far more simple. Things really do go bump in the night. But occasionally, the culprit is closer to home than we might imagine.

Falling asleep is not the neurological equivalent of switching off a light. The brain moves gradually from wakefulness into sleep, accompanied by changes in sensory processing, muscle activity and awareness of the world around us.

But these changes don't always occur in perfect synchrony. So for brief periods, characteristics of sleep and wakefulness can overlap. The results can be surprisingly spooky.

If you've ever been on the verge of sleep only to feel that you've fallen off a curb, missed a step or dropped through the bed, you've probably experienced a hypnic jerk . Also known as a sleep start, this is a sudden involuntary contraction of one or more muscles as we move into the earliest stages of sleep.

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