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