A team led by researchers from the University of Texas Southwestern Medical Center at Dallas and including Northwestern University’s Joseph S. Takahashi, used mice lacking the Clock gene to examine the possible involvement of the biological clock system in the rewarding properties of cocaine. (In 1997, Takahashi led the team that cloned Clock, the first mammalian circadian gene to be cloned.)

In the study, mice that lacked the Clock gene were injected with cocaine. Not only did the mice experience problems with their circadian cycles — not sleeping as much and becoming more hyperactive — they also found cocaine more rewarding than control mice, demonstrated by their strong preference for the location where the drug was administered.

In addition, Clock-deficient mice produced more dopamine than control mice did, suggesting that the gene controlling circadian rhythms is a key regulator of the brain’s reward system and may influence the addictive properties of drugs such as cocaine. (Dopamine is a neurotransmitter associated with the “pleasure system” of the brain, providing feelings of enjoyment from certain activities.)

From a Northwestern University press release.