Ah, a mystery solved.

I wondered why my phone didn't get a full charge overnight; it almost always does. So I checked the graph in the battery settings.

Why? Because I went to bed so late but woke up so early! My phone did not get a full charge in that short time.

This week I have slept around 4 hours a night, frequently interrupted by an insistent pager.

Tonight's plan: both my phone and I will recharge fully.