I have been told that you shouldn't sing until you have been awake for a least 3 hours. I try to follow this rule, but sometimes being up in time would mean not sleeping enough, and lack of sleep is also not great for the voice. It's also hard to know how rigid you need to be with these rules, but it seems I will have to try harder to follow them.
On Saturday it was obvious that my voice was not as flexible as usual. I was happening problems with both the speed that I needed to sing, and also properly placing a couple of the notes. I was in a rush, hadn't slept enough, and hadn't been awake long enough. Not a good combination as at times the strain could be heard in my voice. And if the strain can be heard, it's hardly doing anything good for my throat. I will be singing again this evening, but at least this time I should have had enough rest.