Look into one of the following books and medical articles to read more about light therapy and the working of light to make you feel more energetic:

Books:

Winter Blues. Seasonal Affective Disorder. What it is and how to overcome it.
Rosenthal, Norman E. Guilford Press, 1998 ISBN: 1-57230-395-6.

Scientific articles:

Bright Light improves vitality and alleviates distress in healthy people
Timpo Partonen, Jouko Lönnqvist.
Journal of affective disorders 57, 2000

Therapy and prevention of winter depression. The Importance of the Timing of the Exposure to Light.
Meester Y.
Thesis, Groningen, 1994

Seasonal Affective Disorder. The clinical picture and the effects of bright light treatment in winter depression.
Partonen T.
Thesis, Helsinki, 1995

Phototherapy in Individuals With and Without Subsyndromal Seasonal Affective Disorder.
Kasper S., Rogers S.L., Yancey A.L., Schulz P.M., Skwerer R.G., Rosenthal N.E.
Arch Gen Psychiatry 46, 1989

Light Therapy for Seasonal Affective Disorder, A Review of Efficacy.
Terman M., Terman J.S., Quitkin F.M., McGrath P.J., Stewart J.W., Rafferty B.
Neuropsychopharmacology 2, 1989

Bright Light therapy of subsyndromal seasonal affective disorder in the workplace: morning vs. afternoon exposure.
Avery DH.
Acta Psychiatrica Scand., 2001

Light Therapy in Seasonal Affective Disorder is Independent of Time of Day or Circadian Phase.
Wirz-Justice A., Graw P., Kräuchi K., Gisin B., Jochum A., Arendt J., Fisch H.U., Buddeberg C., Pöldinger W.
Arch Gen Psychiatry 50, 1993