In my case, that would be negative returns.

When i'm tired, i'm basically more productive doing nothing than when i'm touching code.

Also, for a few hours after i'm forced by my body to take my pain meds for bad back (usually once a month on average). And being on the autistic spectrum, there are sometimes days my brain just refuses to do anything productive and i just can't concentrate on one singel thing for more than a minute or so.