Polysomnography: What Happens During a Sleep Study and How Results Are Interpreted

When you’re exhausted every day but can’t figure out why, it’s not just stress or bad habits. It could be something deeper - something your body shows during sleep but you never see. That’s where polysomnography comes in. It’s not a fancy gadget or a quick home test. It’s the most detailed sleep study doctors use to find out what’s really going on when you’re asleep.

What Exactly Is a Polysomnography?

Polysomnography, or PSG, is a full-night sleep study that records how your body behaves while you sleep. The word breaks down simply: poly means many, somno means sleep, and graphy means recording. So, it’s a multi-parameter sleep recording. And that’s the key - it doesn’t just check your breathing. It watches your brain, eyes, heart, muscles, oxygen levels, and even how you move through the night.

This isn’t something you can do at home with a wearable device. It’s done in a sleep lab, usually in a quiet, dim room that’s kept at a comfortable 68-72°F (20-22°C). You’ll sleep on a regular bed, not a hospital gurney. The goal is to make it feel as normal as possible - even if you’re wired up like a sci-fi character.

What Gets Measured During the Study?

A sleep technologist attaches around 22 sensors to your body. Most feel like small stickers or soft bands. You won’t feel pain, but you might notice them. Here’s what each one tracks:

• EEG (electroencephalogram): Electrodes on your scalp record brain waves. This tells the doctor which sleep stage you’re in - light sleep, deep sleep, or REM. That’s critical because disorders like narcolepsy mess with this cycle.
• EOG (electrooculogram): Sensors near your eyes track movement. Rapid eye movement signals REM sleep, the stage where dreaming happens.
• EMG (electromyogram): Small sensors on your chin and legs measure muscle tone. Twitching, grinding, or kicking during sleep shows up here.
• ECG (electrocardiogram): Monitors your heart rhythm. Irregular beats during sleep can point to heart issues linked to sleep apnea.
• Respiratory effort belts: Two elastic bands around your chest and abdomen detect if you’re trying to breathe - even if air isn’t moving.
• Airflow sensors: A thin tube under your nose measures whether air is flowing in and out.
• Pulse oximeter: A clip on your finger tracks how much oxygen is in your blood. If it drops below 90%, it’s a red flag.
• Body position sensor: Tells if you sleep on your back, side, or stomach. Many people breathe better on their side.
• Audio and video: Cameras and microphones record snoring, gasping, or strange behaviors like sleepwalking or yelling.

All of this data streams into a computer in an adjacent room. A sleep technologist watches your night in real time. If you roll off the bed, snore loudly, or stop breathing for 10 seconds, they’ll note it - and sometimes even talk to you through an intercom to help you relax.

How Is It Different From a Home Sleep Test?

Home sleep apnea tests (HSAT) are cheaper and easier. But they’re limited. They usually only track three things: breathing effort, airflow, and oxygen levels. That’s enough to catch simple obstructive sleep apnea - but nothing else.

Polysomnography catches the full picture:

• It can diagnose narcolepsy - where you fall straight into REM sleep instead of going through the normal stages.
• It spots sleepwalking, night terrors, or seizures that happen during sleep.
• It tells the difference between obstructive apnea (airway blocked) and central apnea (brain doesn’t tell you to breathe).
• It measures how much deep sleep and REM sleep you get - key for feeling rested.

Studies show home tests fail to give usable data in 15-20% of cases. In-lab polysomnography fails less than 5% of the time. If you have symptoms like daytime fatigue, unexplained insomnia, or sudden muscle weakness, a home test won’t cut it. You need the full study.

What Happens During a Split-Night Study?

Sometimes, the tech sees severe sleep apnea early in the night - say, over 30 breathing pauses per hour. Instead of sending you home and making you come back, they might do a split-night study.

The first half is diagnostic: they record your sleep to confirm apnea. Once they see it’s serious enough, they wake you up, fit you with a CPAP mask, and start adjusting the air pressure. The second half of the night is therapy. They find the right pressure to keep your airway open.

About 35% of polysomnography studies now include CPAP titration. That saves time, money, and the stress of a second visit. It’s only done if your apnea is moderate to severe - not for mild cases.

How Do You Prepare for the Test?

There’s no fasting. No needles. But you do need to follow a few simple rules:

• Stick to your normal sleep schedule for at least 3 days before the test. Don’t try to “catch up” on sleep.
• Avoid caffeine after noon. That includes coffee, tea, chocolate, and soda.
• Don’t nap during the day. You need to be tired enough to fall asleep at night.
• Wash your hair and skin - no lotions, oils, or makeup. They can interfere with sensor adhesion.
• Bring comfortable sleepwear, toiletries, and anything you need for a good night’s rest - like your own pillow.

You’ll arrive at the sleep center 1-2 hours before your usual bedtime. The setup takes 30-45 minutes. The tech will explain everything. They’ll check that all sensors are working. You can ask questions. You’re not alone.

What Do the Results Show?

After the study, a board-certified sleep doctor spends 2-3 hours analyzing the data. That’s over 1,000 pages of raw information.

The report breaks down:

• Sleep efficiency: How much time you actually slept vs. time spent in bed. Below 85% suggests trouble falling or staying asleep.
• Apnea-hypopnea index (AHI): Number of breathing pauses per hour. Less than 5 is normal. 5-15 is mild, 15-30 is moderate, over 30 is severe.
• Oxygen drop index: How often your blood oxygen fell below 90%. More than 5 drops per hour is concerning.
• Sleep architecture: Percentage of time spent in each sleep stage. Too little deep sleep or REM = unrefreshing sleep.
• Leg movements: If you have periodic limb movement disorder, it shows up here.
• Abnormal behaviors: Sleepwalking, talking, or acting out dreams may point to REM sleep behavior disorder.

The doctor doesn’t just look at numbers. They look at patterns. Did you only stop breathing on your back? That’s positional apnea. Did you enter REM sleep within 10 minutes of falling asleep? That’s a sign of narcolepsy. Did your oxygen drop every time you snored? That’s obstructive apnea.

Who Needs This Test?

You don’t need a polysomnography just because you snore. But if you have:

• Witnessed breathing pauses during sleep
• Excessive daytime sleepiness that doesn’t improve with more sleep
• Unexplained insomnia or frequent nighttime awakenings
• Morning headaches or dry mouth
• High blood pressure that’s hard to control
• History of stroke, heart disease, or obesity
• Symptoms of narcolepsy - sudden muscle weakness, hallucinations at sleep onset
• Strange behaviors during sleep - kicking, yelling, getting out of bed

…then your doctor should consider a polysomnography. Medicare and most private insurers cover it if your symptoms match clinical guidelines. You’ll need a referral and documentation of your symptoms.

What About Comfort and Anxiety?

Most people worry they won’t be able to sleep with all the wires. But here’s the truth: you don’t need a full 8 hours of perfect sleep for the test to work. Even 4-5 hours of recorded sleep can give doctors enough data to make a diagnosis.

About 85% of patients complete the study successfully. Technologists are trained to help you relax. They’ll check in quietly. You can ask for a blanket, water, or to use the bathroom. Many patients say the hardest part is the setup - not the night itself.

If you’re really anxious, talk to your doctor. Some centers offer a practice night - you come in early, get hooked up, and spend an hour watching TV or reading before bed. It helps.

What Happens After the Results?

Your sleep doctor will schedule a follow-up to explain everything. If you have sleep apnea, they’ll recommend CPAP therapy. For narcolepsy, medication and scheduled naps may help. If you have periodic limb movements, they might suggest iron supplements or specific medications.

In some cases, they’ll recommend another test - like a Multiple Sleep Latency Test (MSLT) - to confirm narcolepsy. That’s a daytime test where you’re asked to nap every 2 hours. It’s not scary, but it’s long.

The goal isn’t just to label your problem. It’s to fix it. And polysomnography is the only way to get the full picture.

The Future of Sleep Studies

Newer systems use wireless sensors. Instead of 20 wires, you might only have 5-7. That’s less hassle and more comfort. Some labs now use AI to help analyze data faster - spotting patterns humans might miss.

But the core hasn’t changed. You still need the full set of measurements. Home tests are improving, but they still can’t replace in-lab polysomnography for complex cases.

The American Academy of Sleep Medicine says this will remain the gold standard through at least 2030. Because when your sleep is broken, you need more than a guess. You need the truth - recorded, measured, and understood.