When you don’t feel great for a few days in a row, respect that by resting or doing something less intense. I’ve learned never to combine intense exercise with poor sleep or lack of sleep. That really does a number on both your immune system and appetite.
If you’re healthy and well-rested but just a little mopey or down, a workout can be a great mood boost. If you’re exhausted, very stressed, sleep-deprived, or the thought of intense exercise fills you with dread, the couch and a blanket is a better course of action. Other options would be going for a walk instead of a run, stretching instead of lifting, or doing five minutes of basic body weight exercises instead of a longer, more challenging workout. That way you’ve taken some time for yourself, done some enjoyable movement, but you’re not piling up stressors like when stacking a poor night of sleep with a brutal workday with a challenging run.
A little stress helps us to adapt and grow stronger, too much just breaks things. It’s also so important not to confuse a cortisol stress buzz (fight or flight) with “energy.” Things like going too long without eating or pushing too hard in the gym can give us a wired, invincible feeling that is not at all the same as being legitimately well-rested.