Day and night
DAY AND NIGHT
Sigil has no sun or moon, but it still has a day-night cycle. Each twenty-four-hour period slides from deep darkness to bright light and back again on a regular cycle. The sky gradually fills with a magical luminescence until the light reaches its brightest intensity (the equivalent of noon in the natural world), and then it begins to fade.
At daybreak, the darkness gradually brightens to dawn, with the light increasing in intensity until midday. The light then steadily wanes into a twilight gloom before plunging into blackest night. Unless the night is particularly overcast or foggy, some ambient light shines down from above. This is not the light of distant stars or a moon, since Sigil has none of these.
Instead, the fires burning in the streets and buildings above light the night sky with flickering pinpoints of brightness.
The overall cycle provides six hours of bright daylight and six hours of deepest night, each separated by a six-hour period of gloomy twilight.