References on Light and the Biological clock

Phase Shifting Human Circadian Rhythms: Influence of Sleep Timing, Social Contact, and Light Exposure. Journal of Physiology 1996; 495(1):289-287
J.F. Duffy, R.E. Kronauer, C.A. Czeisler.

"Circadian Rhythms are biological oscillations with periods of close to, but not exactly 24 h. These rhythms are entrained or synchronized to the 24 h geophysical day by signals from the environment. The timing of the rest-activity schedule, social interactions and exposure to light have all been hypothesized to entrain circadian rhythms to the 24 h day. The role of light as the primary synchronizer has been demonstrated in a variety of experiments."
"The present studies reaffirm the powerful effect of the light-dark cycle in circadian rhythms in humans." Full study

Hormonal and Pharmacological Manipulation of the Circadian Clock: Recent Developments and Future Strategies. Sleep 2000; 23 Suppl 3:S77-85
G.Richardson, B Tate.

"But in a modern world where the constraints of environmental time are less and less important, the circadian clock still imposes rigid boundaries on the timing of sleep and alert wakefulness that are increasingly perceived as limitations on human performance. This conflict underlies the sleep 'disorders' of jet lag and shiftwork sleep disruption, problems that are not really diseases at all, but instead reflect normal function of circadian timing in the context of extraordinary demands on sleep-wake scheduling." Abstract

Light Entrainment of the Mammalian Biological Clock. Progress in Brain Research 1996; 111:175-190
J.H. Meijer, K. Watanabe, L. Detari, M.J. deVries, L. Albus, J.A. Treep, J. Schaap, and W.J. Reitveld.

"Mammalian circadian rhythms are controlled by a pacemaker which is located in the suprachiasmatic nucleus (SCN) of the hypothalamus. The SCN produces rhythms of roughly 24 h. In the external world, circadian rhythms are entrained to the environmental light/dark cycle and adopt the external period of 24 h." Abstract

The Anatomical Basis for the Expression of Circadian Rhythms: The Suprachiasmatic Nucleus (SCN) has a Key Role in the Synchronization of a Series of Behavioral, Metabolic and Endocrine Functions to the Daily Light-Dark Cycle. . Progress in Brain Research 1996; 111: 229-240.
R.M. Buijs

"The role of the SCN as a master synchronizing system in the brain of mammals has been demonstrated in a number of successive studies, of which the one by Moore and Eichler (1972) was the first to point to the SCN and the one by Ralph et al. (1990) finally completed the evidence for the SCN as a daily synchronizing system." Abstract