I think I've mentioned before that I used to put natural spectrum florescent lights (with more blue wavelengths) in my research laboratory, because I found it promoted my relaxation and alertness, and my graduate students and post-docs reported the same effect. Chellappa et al. offer yet another study that documents this effect, and relates it to suppression of melatonin levels by blue wavelengths. The observations can be made by simply comparing commercially available compact fluorescent lamps that provide correlated lamp colour temperature in kelvin (K), that indicate the relative proportion of warm versus cool colours in a light source. The authors found that 2-hour exposure to light in the evening with compact fluorescent light at 6500K (blue shifted) will attenuate the expression of endogenous melatonin levels, and also promote an augmentation of subjective and objective alertness levels when compared with lights at 2500K and at 3000K (more yellow). The light with more blue wavelengths has the overall effect of enhancing alertness and performance in cognitive tasks specifically associated with sustained attention.