Intraocular pressure (IOP), or eye pressure, is the most important risk factor for glaucoma. It is also the only risk factor that treatment can change. Lowering IOP slows glaucoma progression regardless of the starting pressure level. But what many patients and even some doctors underestimate is this: a single pressure reading in the clinic does not capture the full picture. Eye pressure varies: through the day, through the night, and across weeks and months. That variation, not just the absolute number, drives glaucoma damage.
Dr Shibal Bhartiya is a fellowship-trained glaucoma specialist and Mayo Clinic Research Collaborator with over 25 years of experience. Her approach focuses on identifying risk before damage is irreversible, simplifying treatment decisions, and protecting vision long-term. Emphasis on early detection, risk assessment, and continuity of care. Her special interest in research has been 24 hour IOP monitoring. She is rated 5 stars across 1,500+ patient reviews on Google.
How Does Eye Pressure Vary?
Eye pressure is not a fixed number. It rises and falls over hours and days. Both the peak level and the degree of fluctuation matter. Short-term spikes and long-term variation are each independently linked to glaucoma progression. This is why your doctor may check your pressure several times at different hours, or arrange for monitoring across a longer period.
When Is Eye Pressure Highest?
For most people, eye pressure peaks between 2 AM and 4 AM. The body produces its highest levels of cortisol, a hormone that influences IOP, during these hours. In glaucoma patients, the gap between daytime and night time pressure is often wider than in healthy eyes. This nocturnal spike is invisible during a standard daytime clinic visit.
Why Does 24-Hour IOP Monitoring Matter?
Dr Bhartiya and colleagues have published peer-reviewed research on this question. A 2019 review, 24-hour Intraocular Pressure Monitoring: the Way Ahead (Bhartiya S et al., Romanian Journal of Ophthalmology, PMID: 31915728), established that the Goldmann applanation tonometer, the standard clinic instrument, cannot measure night IOP without disturbing sleep, leaving a critical blind spot in glaucoma management.
The clinical consequences are significant. Peak IOP occurs outside office hours in close to half of all glaucoma patients. Treatment plans change in more than 35% of patients once 24-hour IOP data is available. Diurnal IOP variation is an independent risk factor for visual field loss, separate from the average pressure level itself.
A second publication, Diurnal Variation of IOP in Angle Closure Disease: Are We Doing Enough? (Bhartiya S et al., Romanian Journal of Ophthalmology, PMID: 31687621), examined this specifically in angle closure glaucoma patients. The research found that many management decisions in glaucoma are made after only one or two IOP measurements, which captures neither the peak nor the full range of pressure behaviour, and that this is a particular problem in angle closure disease, where pressure spikes can be steep and unpredictable.
A third paper, Comparative Evaluation of TonoPen AVIA, Goldmann Applanation Tonometry and Non-Contact Tonometry (Bhartiya S et al., International Ophthalmology, PMID: 21761249), showed that different instruments cannot be used interchangeably in serial monitoring. Central corneal thickness influences all pressure readings. Switching tonometers between visits introduces error that can mask real pressure changes or create false alarms.
IOP fluctuation is a particular concern in angle closure disease, where pressure spikes can be steep and are frequently missed by routine daytime readings. Dr Bhartiya’s published research has examined this directly. A 2015 study in the Journal of Current Glaucoma Practice, Diurnal Intraocular Pressure Fluctuation in Eyes with Angle-Closure (Bhartiya S, Ichhpujani P; PMID: 26997828), investigated IOP fluctuation across the day in 77 newly diagnosed angle closure patients and documented the range and pattern of diurnal variation in this group. A 2019 review in the Romanian Journal of Ophthalmology, Diurnal Variation of IOP in Angle Closure Disease: Are We Doing Enough? (Bhartiya S et al.; PMID: 31687621), went further — finding that many clinical decisions in angle closure glaucoma management are based on only one or two IOP measurements, and arguing that this is insufficient given the established circadian rhythm of IOP and its direct correlation with glaucoma progression. Taken together, these papers make the case that angle closure patients may be among the most undertreated precisely because their worst pressure moments are the least observed.
Options for 24-Hour IOP Monitoring
Hospitalisation for IOP Monitoring
Your doctor may arrange inpatient monitoring, where eye pressure is measured at regular intervals through the night. This captures the nocturnal peak and documents the full diurnal curve.
Patient Self-Tonometry
Some specialists provide a home tonometry device and train patients to measure their own eye pressure at scheduled times. This records pressure variation across normal daily activities, including exercise and sleep positions.
Sleep Laboratory
In selected patients, pressure monitoring during a formal sleep study allows simultaneous recording of body position, sleep stages, and IOP: particularly useful in normal tension glaucoma where vascular and positional factors may contribute.
The Triggerfish® Contact Lens Sensor
The Triggerfish® (Sensimed) is a specialised contact lens with a built-in microsensor. It is worn for 24 hours and continuously records IOP-related changes, including during sleep, without disturbing the patient. It does not measure pressure in mmHg directly but detects circumferential corneal deformation patterns associated with IOP fluctuation: capturing peaks and spikes that a clinic tonometer cannot.
Dr Bhartiya was the first doctor in India to use the Triggerfish® contact lens sensor in clinical practice. Her initial research examined IOP-related patterns in patients with primary angle closure (PAC) and primary angle closure glaucoma (PACG) before and after laser peripheral iridotomy (LPI). These findings were presented at ARVO 2014 in Orlando, Florida. This work established that IOP fluctuation patterns in angle closure disease differ meaningfully from open angle disease, and that laser treatment changes those patterns in ways that standard clinic measurements do not reflect.
The Water Drinking Test
The water drinking test is a simpler, low-cost method for estimating peak IOP. Your doctor measures your eye pressure, asks you to drink roughly 10 ml of water per kilogram of body weight over five to ten minutes, then measures pressure every 15 minutes for an hour. The result gives an approximate estimate of nocturnal peak pressure and short-term IOP variability. It is not a replacement for 24-hour monitoring but is useful when formal monitoring is not available.
What Should You Do If Your Glaucoma Is Progressing?
If your glaucoma is worsening despite eye pressures that look controlled in the clinic, nocturnal IOP is the first question to ask. Office-hour readings may look acceptable while night pressures — the ones doing the damage — remain uncontrolled. This is one of the most common and most underrecognised causes of progression despite treatment.
Key Points to Remember
Night pressure is almost always higher than daytime pressure. A single clinic reading does not tell the full story. IOP variation — not just peak IOP — is an independent driver of glaucoma damage. More than a third of treatment plans need revision once 24-hour monitoring is done. If your glaucoma is progressing despite controlled daytime pressures, ask specifically about night-time IOP assessment.
Frequently Asked Questions
Why is one eye pressure reading not enough for glaucoma management?
Eye pressure varies through the day and night. A single daytime reading misses the nocturnal peak, which is when pressure is usually highest. IOP fluctuation across 24 hours is an independent risk factor for visual field loss. Clinical decisions based on one reading can underestimate true pressure load.
When is eye pressure highest during the day?
For most people, eye pressure peaks between 2 AM and 4 AM due to cortisol release. The difference between daytime and nighttime pressure is larger in glaucoma patients than in healthy eyes.
What is the Triggerfish® contact lens sensor?
The Triggerfish® is a wearable contact lens with a built-in microsensor that records IOP-related changes continuously over 24 hours. It detects pressure fluctuations and nocturnal spikes that standard clinic measurements cannot capture. Dr Shibal Bhartiya was the first doctor in India to use it in clinical practice.
What is the water drinking test in glaucoma?
The water drinking test estimates peak eye pressure. The patient drinks approximately 10 ml of water per kg body weight over five to ten minutes. Eye pressure is then measured every 15 minutes for an hour. The test gives an approximation of nocturnal peak IOP and is a low-cost alternative when 24-hour monitoring is unavailable.
Can different tonometers be used interchangeably for serial IOP monitoring?
No. Research by Dr Bhartiya and colleagues showed that TonoPen, Goldmann applanation tonometry, and non-contact tonometry cannot be used interchangeably in serial monitoring. Switching instruments between visits introduces measurement error. Corneal thickness also influences all readings and must be accounted for.
What should I do if my glaucoma is worsening despite normal clinic pressures?
Ask your doctor specifically about 24-hour or nocturnal IOP monitoring. Office-hour pressures may appear controlled while night pressures remain elevated — which is one of the most common causes of unexplained glaucoma progression.
Read the research articles.
This article was written by Dr Shibal Bhartiya, fellowship-trained glaucoma specialist and Mayo Clinic Research Collaborator, Clinical Director at Marengo Asia Hospitals, Gurugram, known for ethical, patient-centred glaucoma care and independent glaucoma second opinions. This article was edited in April 2026.
She has published peer-reviewed research on glaucoma management, examining how treatment decisions should balance medical evidence, patient preferences, and long-term vision outcomes.
As Editor-in-Chief of Clinical and Experimental Vision and Eye Research and Executive Editor of the Journal of Current Glaucoma Practice (PubMed Indexed, official journal of the International Society of Glaucoma Surgery), Dr Shibal Bhartiya brings editorial and research depth to every clinical decision. Her 200+ publications, including 90+ PubMed-indexed publications and 28 edited textbooks span glaucoma biology, surgical outcomes, health equity, and emerging diagnostics.
Her work can be accessed on PubMed, Google Scholar, ResearchGate and ORCID.
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