The same resident is a different cohort at 4am.
Cohorts aren't fixed to a person — they're fixed to a person and a time. The resident who is oriented and chatty at 10am is agitated and disoriented at sunset. Time-of-day-aware behavior for sundowning.
At 10am the resident in room 214B is good company. She knows where she is, jokes about the breakfast, asks Companion about the weather. At 4:30pm, as the light goes flat, a different person is in the room. She's restless. She wants to go home to a house she sold a decade ago. She's looking for children who are grown. This is sundowning, and it taught us that a cohort is not a property of a resident — it's a property of a resident at a time of day.
Temporal cohorts
Everything earlier in this series treated a cohort as a stable label: this resident is hard-of-hearing, that one has aphasia. Sundowning doesn't fit. The agitation rises in the late afternoon and evening, peaks, and clears by morning. So the same device, for the same resident, has to behave as one cohort in the morning and another at dusk. We model that explicitly as a temporal cohort — a tuning profile bound to a window on the clock, not just to a person.
Mechanically it's a scheduled override. The base cohort config lives in Firestore; a time-of-day layer sits on top, and the API server swaps the active profile as the resident's local clock crosses into the sundowning window. The window itself is per-resident, because sunset and routine differ, and because the onset time is itself something worth tracking.
What changes at dusk
When the evening profile is active, Companion's whole posture shifts:
- Calmer, slower, lower-energy voice — matching agitation with brightness pours fuel on it.
- Heavier reorientation to the concrete and present — the room, the time, the next meal — and no debating the trip home.
- A lower threshold to involve a person, because evening agitation is the leading edge of exit-seeking and falls.
- Less novelty, more reassuring repetition; the looping-answer policy from the dementia cohort gets stricter.
- Quiet logging of onset time and intensity each evening, so a shifting pattern becomes visible.
Designing for the average time of day is the same mistake as designing for the average resident. 4pm and 4am are different rooms.
The clinical value isn't only the calmer evening. It's the record. We store events, not recordings, so each night's onset and intensity become a clean time series. When sundowning starts an hour earlier than it did two weeks ago, that's a trend a nurse can act on — a sign of progression, a new infection, a medication timing that needs review.
At the bedside the effect is that the device meets her where she is, when she is there. The Companion that joked about breakfast is the same one that, at dusk, lowers its voice, brings her back to the room, and quietly raises a hand for the floor nurse before the restlessness becomes a fall.