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Extreme fatigue the day after HIIT

Hi, I've just come back to my workout routine after a couple of months break for travel and then sickness. I've been able to build my stamina back up but am having trouble with the HIITs. I am actually enjoying doing the HIIT and then have super high energy the whole day and feel great. But then the next day, I am just slammed with fatigue. I can barely move and would rather just sleep the whole day through.

I'm now getting worried about the HIIT as I can't afford to be non-functional at work. And if I do it on a Saturday, I lose out on all Sunday.

I eat a proper meal soon after HIIT (usually have dinner within half an hour) and drink a decent amount of water as well.

Advice much appreciated!