I was into “concentration meditation” at the time. Using “the feeling of breath in the tip of my nose” as my object of concentration.
So I was in a dream (I was in a windowless, slightly cluttered basement, talking to some people, thinking about going upstairs) when it occurred to me to concentrate on my (real physical) nose.
The light started strobing. A slow strobe. Like 0.7 hz. The whole light of the dream. Like somebody was flicking the basement light on and off