An improved method for precise control of light exposure at a known circadian time during an animal's subjective night.

The paper describes an electronic device that improves the feedback lighting (LDFB) developed previously. LDFB links environmental lighting conditions to locomotor or other monitored behavior. Subjective Night Light (SNL) has the following advantages over LDFB: it eliminates the multiple transitions between light and dark; it allows for precise control over lighting so that a light signal of variable duration can be phase locked to any phase of the subjective night; it dissociates to a much greater extent any potential cognitive perception of the link between locomotor activity and lighting; and it can be programmed easily. Finally, SNL retains the significant advantage of LDFB in its ability to maintain phase relationship with the endogenous biologic rhythms even in circumstances of phase-shifting or free-running conditions. The SNL system is made from components that can be purchased at most electronics outlets for less than US$100.