Part of your brain never
actually switched tasks.

In 2009, researcher Sophie Leroy published a study asking a deceptively simple question: why is it so hard to do my work right after switching to it? Her experiments had participants work on one task, then switch to a second, unrelated task requiring sustained attention. The consistent finding: people performed measurably worse on the second task when the first one had been left incomplete, as if part of their attention hadn't come along for the switch.

Leroy named that leftover attention "attention residue," and the paper, published in Organizational Behavior and Human Decision Processes, has since been cited more than 500 times. The core idea reframes what a task switch actually costs: it's not just the time to reorient, it's a genuine cognitive carryover, a piece of your mind still working the old problem while you're supposed to be working the new one.

Leroy's research found the effect isn't uniform, it's strongest under specific conditions: when the first task was left unfinished, when it was time-pressured, or when it carried some emotional weight. A task you completed cleanly and closed leaves comparatively little residue. A task you got pulled away from mid-thought, or one you're anxious about, leaves considerably more.

Anticipating a rushed return makes it worse. If part of you already expects you'll have to hurry back to the interrupted task later, that expectation itself seems to keep more attention tethered to it in the meantime, compounding the residue rather than letting it fade.

A phone check is a task switch like any other, and by Leroy's findings, an unresolved one is exactly the kind that leaves residue behind. A message you read but didn't reply to, a feed you scrolled but didn't finish, a notification that raised a question you didn't answer, all of these look like brief interruptions but function as incomplete tasks the same way an unfinished work assignment does.

That's part of why a 30-second phone check can cost meaningfully more than 30 seconds. The time on the phone is the visible cost. The residue left behind, some of your attention still processing whatever you saw, is the cost that shows up afterward, in the work you return to.

Social feeds, in particular, are structurally built to never feel finished, there's no bottom to reach, no natural completion point the way a finished email or closed document has. By Leroy's own framing, that's close to a worst-case setup for residue: a task that's always technically unfinished, that you always leave mid-stream rather than at a clean stopping point.

That's a meaningful part of why returning to work after a social media check can feel foggier than returning after, say, making a cup of coffee. Coffee has a clear endpoint. The feed doesn't, and neither does the small mental thread it leaves behind.

Leroy's own recommendations focus on finishing tasks before switching, or consciously planning the next step before stopping, both of which reduce residue at the source. Neither of those strategies helps with a reflexive phone check, though, since the switch itself wasn't planned in the first place.

Fella reduces the number of available switches by keeping selected apps blocked by default, so there's no reflexive option sitting one tap away all day. One 5-minute emergency unlock a day covers a genuine need, and the block returns automatically when the window ends.