Many physiological functions show day-night fluctuations called circadian rhythm. Day time fluctuation is called diurnal variation
which is a part of circadian rhythm. Both are enabled by the biological clock in supra-chiasmatic nucleus (SCN) of hypothalamus.
Impact of diurnal oscillations on body temperature and metabolic rate are widely studied. However, its role in modulating the lung
functions from morning till evening is less explored. Hence, an attempt had been made in this arena. Respiratory functional status was
assessed at 8 AM, 12 noon and 5 PM in 15 normal young individuals in the age group of 17 to 20 years by pulmonary function test
(PFT).The values of PFT variables were analysed in SPSS version 17 by using ANOVA. The mean values of FVC, FEV1, FEV1/FVC
ratio, FEV25-75% and PEFR were maximum (122.80, 128.20, 121.40 and 107.33)at 5 PM and minimum (105.00, 98.40, 99.53 and
97.80) at 12 noon revealing that lung function was at its best around 5 PM with minimum airway resistance and worst at 12 noon with
maximum airway resistance. It is postulated that SCN regulates circadian rhythm through retinal impulses and pineal gland. Pineal
gland, in turn, controls circadian rhythm by secreting melatonin which may play a key role in modulating the expression of
chemotaxins in airway epithelium. We conclude that asthe pulmonary functional status is at its best around 5 PM it may be considered
in determining the time of treatment to enhance the therapeutic efficacy for the pulmonary disorders