Lose weight as you sleep?

[janewilliams20]

I knew my weight varies during the day, and to get a clear(-ish) picture of progress, I should weigh at the same time each day. Just for a laugh, I weighed myself before going to sleep, then weighed again when I woke up. There's a kilo difference! Yes, I'd lost a kilo overnight (0.2 down from the previous morning, so daily loss is still sensible). I wonder where it's going? No obvious lumps of fat found hiding under the sheets....