If energy, then I'd question your sleep, diet, job stress level, then fitness level (in that order).
If not, then I'd question your device addiction/attention span degredation and then perceived value of alternate activities (i.e. do you REALLY want to read? Or just want to want to read?).
One difference is that if I actually had a real deadline I would deliver no matter how tired/exhausted I am, but I'm not sure that's sustainable.
If you require a deadline to do something I'd wonder if you don't want to do it that badly. Which is okay, for things like work or even the last 20% of a hobby project. But reading.. I try to make that only for enjoyment.
I find more things I do which require will power or a deadline, the more I'm dipping into a finite resource which when expired leads to burnout.