Gtalked about the day after phenomenon from the outside perspective – I thought I’d add mine and how it feels to be in it.
The day after – in non-medical terms obviously, in medical terms it’s called post fatigue or post exercise malaise. A combination of words that always invokes the image of a fragile women in a regency dress lying on a chaiselongue with a damp cloth over her eyes. (something like that, plus the damp cloth, don’t ask about the visual…)
The day after always hits after an exiting day, no matter what you do, whether you did well or not, whether you wasted time or energy or used it well. Specifics don’t matter. You did something that drains your energy levels way below the allowed level, so you will pay for it. There is no escape, no bargain you can strike. Obviously, you will feel a lot better (mentally and emotionally) if you did something you feel was worthwhile rather than not.