Apple’s sleep detection is wrong

I had a delicious lie-in this morning, and extra 40 minutes asleep. When I looked at my sleep stats, however, I had been robbed. Apple says I woke up at 6:59. I happen to know that I did not actually get out of bed until 7:40. I was awake, briefly, at around 6:59, but only to tap the Stop button. Then straight back to sleep and a strange dream that involved Sudoku puzzles and being unable to leave a room. But I digress.

Of course it is simplest to assume that because my alarm went off, I am no longer asleep, but life isn’t always like that. My watch knows when I am standing upright and have been for a couple of minutes. Why can’t something like that signal the end of my sleep (provided it isn’t just disturbance during the night)? Come to think of it, why can't my watch record when it thinks I have had a nap?