The dawn window runs from roughly four in the morning until seven, the stretch where a late night and an early morning stop being distinguishable and start needing different decisions.
At around four something in the character of the night gives way. The small hours have run out, the first traffic has not started, and the light is about forty minutes away in summer and hours away in winter. The stretch from four until seven is different enough from what came before it to be worth naming.
The first has not been to bed and is at the far end of a long evening. The second has been to bed, woken at half past three with a body still on another continent's clock, and is at the beginning of something. They want different lengths and different formats, and the thread works better when the message says which one you are. Both cases are worked through at length in the guide to this window.
Slower things, mostly. A dawn booking that ends with a guest wide awake at seven has solved nothing, and formats that finish quietly suit the window better than formats that finish briskly. Length matters too: an hour that ends at five thirty leaves a strange half of a night, whereas two hours that end at seven arrive at a morning.
The forty to sixty minute arrival window holds here as everywhere in the centre, so a booking confirmed at 04:15 means a knock before five. Availability is at its narrowest across this stretch, narrower even than in the small hours, so flexibility about who comes buys you far more than flexibility about when. Nothing about the fee changes, and the deposit rule is the same one that applies at noon.
At breakfast, in practice. Once a building starts moving, a corridor is no longer a quiet corridor, and a booking that runs into eight in the morning is simply a morning booking with a long history behind it.
Two lines settle it: where the room is, and what time you want someone at the door. The answer comes back with a name, an arrival window and the whole figure, whatever the clock says.