Abstract: A naturalistic illumination system for a user at a location having a light source for providing illumination of variable intensity above a non-zero value to the user location. A computer produces a variable signal to control a cycle of variable intensity illumination level output of the light source to correspond to the progressive variation in light level intensity above a non-zero value over a selected time of day period at a geographical location as selected from a computer input device. The variable signal is produced on a daily basis to produce the variable intensity illumination cycle for the same period of time in a manner to progressively shift the time of the start of the production of the variable intensity illumination cycle for the period of time each day over a period of successive days relative to the normal time of start of the production of the variable intensity illumination cycle during the period of successive days at the selected geographical location.