Some of my most stressful times at work have been when I had the least stuff to do. It's nice for a few days, but when it stretches into weeks I start feeling queasy.
Obviously, being buried under an avalanche of thankless work is just as bad.
My ideal life would be some sprints of large effort (maybe pulling the occasional all-nighter once in a while), followed by rest / low work times.
A constant amount of work, all the same, blurs the days together. Too little, and you start feeling useless. Too much (consistently) and you're overwhelmed.