Extreme fatigue

For sure. But adequate rest, both sleep and waking relaxation, are important too. I frequently 'waste' my days off doing nothing because i am basically constantly sleep deprived. It would be better to get consistently enough sleep every night and establish a long term routine that gives me what i need every day, but sadly that doesnt fit into my lifestyle right now (toddler father). My sleep hygiene in general could use some work tho.

Are you new to the gym or just starting out? Starting or increasing your workouts will tire you out big time.

Are you getting enough sleep generally? Are you suffering from anxiety or stress? Or depression? Is your diet poor, or have you made any big changes to it recently? Do you wake tired or refreshed in the mornings? Have you tried actually sleeping during a daytime nap, is it possible and do you wake up refreshed or worse than before?
Do you usually do nothing on days off? Do you feel more stimulated if you spend a day with others rather than alone? How about in a different place? Or if you were doing something you really enjoy?