Aim & objective: The aim was to find out the ultradian rhythm variation (morning and evening ) in frequency domain indices of HRV in healthy young adults.

Materials and Methods : A total of 100 healthy subjects (50 males and 50 females) in the age group of 18-25 years were included in the study. They were screened by history and physical examination to exclude cardiovascular and any abnormalities. Methodology: Short term HRV (Lead II electrocardiography [ECG] for 5min) was done by using standardized neuroperfect electromyography 2000 (with installed HRV software) machine. For each and every subject two recordings were done, - one in the morning between 9 and 10 AM before starting the daily activities of the day and one in the evening between 3 and 4 PM at the end of the day.

Result:The results were statistically analyzed by using Students t-test. Comparison of frequency domains low frequency (LF), high Frequency (HF), LF/HF ratio between morning and evening sessions for the same individual were carried out. LF was increased in the evening session when compared to the morning session and the difference is statistically significant. ( P < 0.0001). HF was decreased in the evening session when compared to morning session and the difference is not statistically significant ( P=0.2745 ). LF/HF ratio also increased in evening session when compared to morning session and the difference is statistically significant (P < 0.0001).

Conclusion: There exists a significant difference in LF and LF/HF ratio between morning and evening recordings. It is concluded that time of the day has a significant impact on cardiac autonomic tone.