"Just push through" is common advice for a long work session, but it tends to work worse than it feels like it should. Structured breaks — ones you take on a schedule, not when you finally notice you're exhausted — are one of the few productivity habits that hold up under scrutiny, mostly because they work with how attention actually behaves rather than against it.
Sustained focus on one task tends to degrade well before it feels like it's degrading. You don't usually notice the drop in the moment — you notice it later, when you look back at the last hour of work and realize how much of it you'll need to redo or double-check. Waiting for the feeling of fatigue to tell you when to stop means you're already well past the point where a break would have helped.
One reason regularly-timed breaks tend to outperform "work until tired" is the idea of ultradian rhythms — cycles of higher and lower alertness that seem to run roughly every 60 to 120 minutes across the day, distinct from the 24-hour circadian rhythm that governs sleep. The exact length varies by person and by task, which is part of why there's no single "correct" interval length — but the general pattern, alertness rising and falling in waves rather than declining steadily, is a reasonable argument for working in blocks instead of one long, undifferentiated stretch.
Deciding "should I stop now?" is itself a small cognitive cost, repeated every few minutes if you're doing it on willpower alone. A timer removes that decision — the bell rings, you stop, no negotiation. This is much of why the Pomodoro Technique and similar methods work as well as they do for a lot of people: the value isn't really in the specific interval length, it's in outsourcing the stop/start decision to something outside your own judgment in the moment.
Not all breaks recover attention equally. Stepping away from the screen, moving around, or looking at something in the distance tends to leave people more refreshed than switching to a different screen — a different kind of stimulation isn't the same as rest. If the break is only a few minutes, standing up and looking away from your work is usually a better use of it than opening another tab.
LoopBell's Pomodoro mode alternates focus and break periods automatically, so the stop/start decision is never yours to make in the moment — the bell just rings.