Making sense of your sleep study results

A sleep study report is a page of numbers and it is easy to fixate on the wrong one. Here is what each figure means, and why the story only comes together when a doctor reads them alongside how you actually feel. No single number diagnoses anything on its own.

AHI and RDI: your breathing

The AHI (apnoea-hypopnoea index) is the number of times an hour your breathing stopped or became shallow. As a rough guide, under 5 is normal, 5 to 15 is mild, 15 to 30 is moderate, and above 30 is severe. The RDI is a slightly broader version that also counts subtler breathing-related arousals. The report also separates obstructive events (your airway collapses) from central ones (your brain briefly stops sending the signal to breathe), which matter for different reasons.

Oxygen: the ODI and the dips

The ODI (oxygen desaturation index) counts how often your blood oxygen dipped through the night, and the report shows your lowest level and how long you spent below key thresholds. Occasional small dips are normal. Frequent or deep dips, especially below 90 percent, are what make apnoea worth treating, because the strain falls on the heart and blood vessels.

How much you actually slept

This is the part that matters most in insomnia. True total sleep time is how long you were genuinely asleep, not just in bed. Sleep efficiency is that as a percentage of time in bed; healthy is roughly 85 percent or more. Sleep latency is how long you took to fall asleep, and wake-after-sleep-onset is how much you were awake once you had. Low efficiency with long latency or a lot of night waking is the fingerprint of insomnia, even when the breathing numbers are normal.

Your sleep stages

The report estimates time in light, deep and REM sleep. Very roughly, adults spend most of the night in light sleep, with something like a fifth in deep and a fifth in REM, shifting with age. Home studies estimate stages well but not perfectly, so we read proportions and patterns rather than treating a single figure as exact. Thin deep or REM sleep, or a very fragmented pattern, tells us the night was not restorative.

Arousals: the insomnia signal

The arousal index counts how often you were roused, including brief autonomic arousals you never remember. A high arousal load with normal breathing is the objective signature of hyperarousal, the wired nervous system that keeps insomnia going. It is often the most useful number on the page for a poor sleeper who does not have apnoea.

The heart-rhythm flag

Some reports flag an irregular heart rhythm, such as atrial fibrillation or extra beats, picked up from the pulse signal overnight. This is a flag, not a diagnosis. If it appears, we arrange a proper heart tracing (an ECG) to check it, because sleep apnoea and rhythm problems often travel together.

Putting it together

The numbers only mean something in context. The same AHI can matter a lot or a little depending on your oxygen, your symptoms and your heart. A perfect breathing result with low efficiency and high arousals points firmly at insomnia, not apnoea. That is why a doctor reads the whole report against how you feel, and turns it into a plan, rather than handing you a page of figures.

Ready to start?

If you would like a proper look at your sleep, the next step is a short consultation with one of our doctors.

Begin your assessment at this link. Online with a home sleep study if it would help, or in person at Westfield London.