Bed bugs are cryptic. They hide during daylight and venture at night to feed. This is adaptive because their human hosts are typically sleeping when a room is dark. Moving bed bugs are more likely to be detected in the light. In laboratory tests*, bed bugs move minimally in light conditions. Within a short period after the lights go out, bed bug activity rises to a peak, then tapers off toward the time when lights go on.
Under continuous light, bed bugs maintain a free running circadian rhythm of about 26 hours. In the absence of a lights off event, bed bugs will remain inactive for about 2h past their normal activity period, then commence movement. Bed bugs have an endogenous activity rhythm. Bed bugs require an average of 4 days to reset their rhythm of activity after a phase shift of 12 h to a 12 h light : 12 h dark photoperiod.
The endogenous rhythm may be adaptive to indoor living with modern lighting. People may awake and turn on a light in the middle of the night. An endogenous circadian rhythm keeps the activity in sync with the natural light/dark cycle rather than resetting in response to random lights on events that are out of sync.
The bed bug circadian cycle is conditioned by a combination of genes used by Drosophila and mammals. However, bed bugs are missing the genes (CRY1 and Jet) that Drosophila use for light input. If present in bed bugs, these genes could reset the circadian clock in response to a lights on event. This would not be adaptive and may be why these genes are lost in bed bugs.
Alvaro Romero, Michael F. Potter, Kenneth F. Haynes. Circadian rhythm of spontaneous locomotor activity in the bed bug, Cimex lectularius. Journal of Insect Physiology. Volume 56, Issue 11, November 2010, Pages 1516–1522.