Until I was five I shared a bed with my sister, 3½ years older than me. After our parents had switched out the light we would chatter away in the darkness until we fell asleep. But I could never afterwards remember falling asleep. It was always the same: one moment I was talking to my sister in the dark, and the next I was waking up in a sunlit room having been asleep all night. Yet every night there must have come a time when I stopped talking and settled down to sleep. It was incomprehensible to me that I did not experience that, and never remembered it.

When I confided my bafflement to my sister she was dismissive. ‘Nobody remembers it,’ she said in tones of confident finality, as if that were all there was to it. I remained dissatisfied. How does she know? I thought? All that means is she doesn't remember it. I bet she's never talked about it to anybody else. So I set myself to keep a keen watch on myself, so that I would know when I was falling asleep, in the same sort of way as people try to catch the light in a refrigerator going out when they close the door. But it was no use. Everything continued just as before. One moment I would be chattering away to my sister in the darkness on, say, a Monday evening, and the next thing I knew I would be waking up in broad daylight and it would be several hours into Tuesday.