Subjects: Fifteen healthy male volunteers between 17.0 and 29.0 years of age (group average age±SD., 23.8±2.8 years) participated in the study.

Design of the study: Recordings were done on consecutive days. The total period of assessment was 40 min, with a 30-min test period preceded and followed by two 5-min periods. During these two 5-min periods the room was illuminated with white light. For both days of assessment the two 5-min periods were the same, while the test periods were different. For randomization, subjects' names were drawn out in the form of a lottery, to assign the subjects to two groups. The first group of subjects was exposed to blue light during the test period of Day 1 (BL session) and red light during the test period of Day 2 (RD session). For the subjects of the second group the order of the BL and RD sessions was reversed.

Recording conditions: Recordings were done in a sound attenuated cabin (4.0 m x 2.5 m x 3.2 m). Subjects were asked to lie with eyes closed, on a bed which was illuminated by red or blue light using four incandescent, focusing, color bulbs of 40 W each, at a fixed distance of 1.4 m from the subject. During the control periods a single, 60 W, incandescent, white bulb, illuminated the room.

Assessments: The electrocardiogram (EKG), respiratory rate, finger plethysmogram, skin resistance and electroencephalogram (EEG) were recorded using a 10-channel polygraph (Polyrite, Recorders and Medicare, Chandigarh, India). The EKG was recorded  using standard limb lead I configuration and an AC bio-amplifier with 1.5 Hz high pass and 75 Hz low pass filter settings. Respiratory rate was monitored with a thermistor worn as a clip at the nostril. To assess the digit pulse volume (DPV), a photoplethysmogram was placed on the left thumb at the junction of the nail and the skin. Skin resistance was recorded using Ag/AgCl disc electrodes attached to the volar surfaces of the distal phalanges of the right index and middle fingers. Electrode gel (Medicon, Chennai, India) was used and a constant current of 10 microamperes was passed between the electrodes. The signal was processed through a DC preamplifier. The EEG was recorded with Ag/AgCl disk electrodes placed at O1 and O2 positions, according to the standard 10-20 system for electrode placement [7], as a bipolar recording. The blood pressure (BP) was recorded at the beginning and end of the test periods using a standard mercury sphygmomanometer, auscultating over the right brachial artery. The diastolic pressure was noted as the reading at which the Korotkoff sounds appeared muffled.

Data extraction and analysis: The heart rate in beats per minute was obtained by continuously counting the QRS complexes in successive 60-second periods. The rate of respiration  was  similarly  calculated  by counting the number of   respiratory    waves   also   in   successive  60-second epochs  continuously  and  noting  the  respiratory  rate as cycles  per  minute.  The skin resistance trace was sampled every 20 seconds. The amplitude of the digit pulse volume was sampled  from  the peak of the pulse wave at 20-second intervals [8]. The EEG record was visually assessed to detect sleep episodes. For  both the blue and red light sessions, the data of the test and the post periods were compared with those of the respective preceding periods using the t-test for paired data.