: Thought-provoking experimental evidence suggests that perinatal light exposure may imprint circadian clocks with lasting effects on the alignment and the stability of circadian rhythms later in life. Assuming that exposure to light early in life could determine the stability of an individual's circadian system later in life, the present hypothesis proposes that time of year and location of birth (i.e., season and latitude) and thus differential Zeitgeber strengths may be key contributors to a person's susceptibility of developing mood disorders like seasonal affective disorder (SAD) and common internal cancers such as those of breast and prostate. Consequently, when and where people are born might critically predispose them to both mood disorders and internal cancers, and may affect the onset and course of such illnesses. This paper develops a causal framework and presents suggestions for rigorous tests of the associated corollary and predictions. It does not escape our attention that links between the perinatal Zeitgeber strength of light and its effects on the stability of circadian systems later in life could have a role to play in affecting long-term health beyond cancer and mood disorders - mostly in adults but also in children.