The shape of a festival evening is largely determined by a time at which sound must stop. That deadline is set by local authorities and everything else is arranged backwards from it.
The curfew is a licence condition
Events operate under permissions granted by local authorities, and those permissions carry conditions covering noise levels and the hours during which amplified sound is allowed.
Conditions exist because festivals take place near people who live there permanently, and the licence is the mechanism through which those residents' interests are represented.
Breaching them risks fines, and more seriously it risks the licence being refused the following year, which is an existential threat to an annual event.
The schedule is built in reverse
Organisers start from the curfew and work backwards, allocating the headline set, the changeover before it, the preceding act and so on down the day.
Each changeover has a fixed duration determined by how much equipment must be moved, and those blocks cannot be compressed without risking a delayed start.
Multi-stage festivals then stagger these schedules against each other so that audiences can move between stages and so that neighbouring performances do not bleed into one another.
Delays are absorbed by cutting sets
Because the end time cannot move, any delay earlier in the day has to be recovered from the acts that follow.
Stage managers reduce set lengths by a few minutes across several performances, which is why running orders shift and why some acts finish earlier than published.
When a delay is severe, a set is cut short at the curfew regardless of what is being performed, and organisers will cut power if a performer refuses to stop.
Sound is monitored continuously
Licences specify measurable limits at defined points, often at the boundary of the site or at nearby homes, and readings are taken throughout the event.
Engineers work within those limits, which constrains how loud a headline set can be regardless of the equipment available.
Weather complicates the task, because wind direction and temperature change how far sound carries, so a level that was compliant in the afternoon may not be after dark.
Curfews explain some familiar festival features
Silent sets using headphones exist because the restriction applies to amplified sound reaching the surroundings rather than to the event continuing.
Dance stages placed furthest from residential boundaries and enclosed structures are the same solution approached differently, using distance and containment to extend hours.
Urban festivals face the tightest constraints and typically finish earliest, which is why city events are structured around afternoons in a way rural ones are not.