Purpose:  To compare the intraocular pressure (IOP) during 12 hour day-time phasing with one-time supine position IOP. Study Design:  Prospective, observational. Place and Duration of Study:  Glaucoma clinic of a tertiary eye – care center in Rawalpindi, Pakistan, from September 2020 to February 2021. Methods:  Forty two eyes of 21 participants with primary open angle and normal tension glaucoma were included in the study by convenient sampling technique. Three hourly diurnal Applanation tonometry was performed in sitting position from 9:00 am to 9:00 pm. Mean IOP and standard deviations were calculated. One-time supine position IOP was taken by Perkins tonometer at 9:10 pm. Mean IOP at 9:00 pm in sitting position was compared with the mean IOP in supine position. Results:  There were 15 males (71.42%) and 6 females (28.58%) in the study. Means of sitting position IOP during different times of the day ranged between 16.64 and 18.16 mmHg. The mean of IOP measured at 9:00am, 12:00 pm, 3:00 pm, 6:00 pm and 9:00 pm was 17.62 ± 4.04 mmHg. Mean supine position IOP at 9:10 pm was 22.38 ± 5.92 mmHg. Mean difference between supine and sitting position IOP was 4.75 ± 3.65 mmHg. This was statistically significant p-0.001.Direct comparison between sitting and supine IOP at 9:00 pm and 9:10 pm revealed the values to be 17.88 and 22.38 mmHg respectively (p < 0.001). Conclusion:  One-time supine position IOP gives a higher value than any one – time of IOP recorded during 12-hour diurnal phasing performed in sitting position. Key Words:  Intra ocular pressure, diurnal phasing, Primary open angle glaucoma, Normal tension glaucoma.