In  the  present study there was a significant decrease in the rate of respiration when subjects were exposed to blue light and it remained lower in the subsequent period. The diastolic blood pressure was also significantly lower immediately  after  exposure to blue light. There were no changes during the exposure to red light. There was a significant decrease in the digit pulse volume in the control period after exposure to both blue and red light. In  chromotherapyz blue light is used in the management of insomnia, as it is believed to reduce physiological arousal [1]. The  present  results (decreased  breath  rate  and  diastolic blood  pressure)  support this idea. The results also show similar trends to those reported in previous studies which described the effects of blue color when the subject was looking at it (i.e., increased skin resistance, reduced BP). Hence though it was shown that the estimated light transmission through the eyelids is greater for red (5.6%) than for blue light (0.3%), significant effects were seen following eyes closed exposure to blue light in the present study [6].
The digit pulse volume reflects blood flow through the skin. A reduction in digit pulse volume suggests narrowing of cutaneous blood vessels. Reduced skin blood flow was shown to occur when subjects were alert, as while solving arithmetic problems mentally [9] and also in response to alerting stimuli [10]. The change that occurred during the control periods after exposure to both blue and red light, suggested that the subjects were more alert during the post periods of both sessions compared to the respective baseline. The decrease in the digit pulse volume suggests peripheral vasoconstriction and hence an increase in peripheral vascular resistance. It is known that increases in peripheral resistance increase the diastolic blood pressure [11]. However these results are not contradictory to the decrease in the diastolic blood pressure immediately after exposure to blue light, as the changes in digit pulse volume occurred in the control period while being exposed to white light.
In summary, the results suggest that blue light reduces physiological arousal, whereas red light does not have this effect. While the exact mechanism by which color can influence physiological functions is not known, it is thought that following cortical interpretation of the stimulus, the affective response determines the autonomic change [12]. The results support the claim that blue light can be used to induce physiological rest, but they do not support the idea that red light has a stimulating effect.