The Myth of 9-to-5 Writing: Why Butt in the Chair May Not Work by Nikki Stern at Talking Writing describes Stern's experience with managing writing time. After having to start getting up and moving every hour because of osteoarthritis, she noticed that she was coming back to work sharper after the breaks, sharper than when she was "pushing through" and putting her butt in a chair for the 9 to 5 hours she'd expected to put in writing.
Stern refers to Tony Schwartz. "Schwartz believes the focused ninety-minute approach is the optimal way
to work productively. He cites classic studies by sleep researcher
Nathaniel Kleitman—particularly Kleitman’s 1960s observations of the
basic rest-activity cycle (BRAC)—as the biological basis for
recommending that workers take a break to rest and refresh every ninety
minutes." We've talked about Schwartz's ninety-minute thing before here.
Ninety minutes, folks. That's a unit of time.
One of the interesting things about breaking your work time into units, whether they are ninety minutes long, forty-five minutes, twenty minutes, or something else, is that there is research, such as that cited above, to support it. I haven't seen any research about butt in chair.
An unrelated interesting note from Stern's essay: She says that a C. Northcote Parkinson came up with the expression “work expands so as to fill the time available for its
completion” in the 1950s. Betty Friedan said that about housework in The Feminine Mystique at a later period, something I've never forgotten. Presumably she was paraphrasing Parkinson and so I have been, too, all this time?