All colas contain caffeine (so should be avoided before sleep). Diet colas may be even more problematic for two reasons: Sugar, the carbohydrate that would help exert a sedating influence, is absent; the artificial sweeteners act as a direct stimulant to the brain. (Guiffre, Kenneth, MD, with Theresa Foy DiGeronimo. The Care and Feeding of Your Brain. p 114. NJ: Career Press, 1999.)