This group studies Korean normo-tension glaucoma (NTG) patients over a 24-hour period to assess if there was a relationship between nocturnal intraocular pressure (IOP) and diurnal IOP. They have previously suggested differences in diurnal IOP may have varying mechanisms of glaucomatous damage. Hence, they set out to assess if there was any difference in nocturnal IOP for patients with low diurnal IOP (mean IOP<15, n=70) and high diurnal IOP (mean IOP≥15, n=79). The study was prospective with age, axial length and disease severity matched patients and a power calculation was performed. The IOP measurements were taken using a single operator using a TonoPenXL which was validated with a previous study against Goldmann applanation tonometry (GAT). Eight measurements over a span of 24 hours were taken from the admitted patient. The study found nocturnal IOP in the low IOP group was significantly higher than that in the waking hours (12.8 vs 14.21 mm Hg P<0.001). This was not replicated in the high diurnal IOP group (16.33 vs 16.40 P=0.706). Furthermore, they found nocturnal IOP elevation was negatively correlated with axial length (p<0.001). Overall, the authors suggest the difference between their groups and nocturnal IOP elevation would mean a more tailored approach towards the timing of medication administration is beneficial in treating NTG in these groups. However, as the TonoPen readings were 95% of the time within 2mmHg of the GAT, it is hard to determine whether a 2mmHg nocturnal elevation is clinically significant.