The Jargon File (version 4.4.7, 29 Dec 2003):

night mode
 n.

    See phase (of people).


The Free On-line Dictionary of Computing (30 December 2018):

phase
day mode
night mode

   1. The offset of one's waking-sleeping schedule with respect
   to the standard 24-hour cycle; a useful concept among people
   who often work at night and/or according to no fixed schedule.
   It is not uncommon to change one's phase by as much as 6 hours
   per day on a regular basis.  "What's your phase?"  "I've been
   getting in about 8 P.M. lately, but I'm going to wrap around
   to the day schedule by Friday."  A person who is roughly 12
   hours out of phase is sometimes said to be in "night mode".
   (The term "day mode" is also (but less frequently) used,
   meaning you're working 9 to 5 (or, more likely, 10 to 6).)
   The act of altering one's cycle is called "changing phase";
   "phase shifting" has also been recently reported from Caltech.

   2. "change phase the hard way": To stay awake for a very long
   time in order to get into a different phase.

   3. "change phase the easy way": To stay asleep, etc.  However,
   some claim that either staying awake longer or sleeping longer
   is easy, and that it is *shortening* your day or night that is
   really hard (see wrap around).  The "jet lag" that afflicts
   travelers who cross many time-zone boundaries may be
   attributed to two distinct causes: the strain of travel per
   se, and the strain of changing phase.  Hackers who suddenly
   find that they must change phase drastically in a short period
   of time, particularly the hard way, experience something very
   like jet lag without travelling.