Nadia could not sleep because she was thinking about consciousness.
Specifically: if she was unconscious when she slept, how did she know she had slept at all? And if she didn't know — if sleeping Nadia and waking Nadia were, in some important sense, different people — then what was the thread that connected them?
This was an unusual problem for eleven o'clock on a Tuesday.
She had read about this once, in a book about philosophy. Something about continuity of identity. She couldn't remember the name of the philosopher. It frustrated her enormously.
She went to her parents' bookshelf, which was tall and slightly chaotic, and found, eventually, a book with a chapter on personal identity. She read it sitting on the landing in the dark, with the hall light on.
The philosopher's answer, as far as she could understand it, was: the thread is memory. You know you are the same person who went to sleep because you remember being them.
Nadia thought about this.
Memory, then. Not the sleeping. Not the waking. The remembering.
She thought about what she would remember of today. Small things, probably. The colour of the light in the afternoon. A conversation. The smell of dinner.
The memories stitched her days together into something continuous. Into a life.
She went back to bed feeling more settled than she had in some time.
She fell asleep almost immediately.
And in the morning, she remembered thinking about it.