During REM sleep there is a type of paralysis or loss of smooth muscle tone in the arms & legs, throat, lungs & diaphragm.
As a result, unless you have REM behavior disorder it is impossible to:
act out your dreams OR
take a deep breath
The result is shallow breathing known as hypoventilation and it causes:
a drop in oxygen levels AND
an increase in carbon dioxide levels
Hypoventilation often occurs with the following medical conditions:
lung disease from COPD, interstitial or pulmonary fibrosis
obesity or late stages of pregnancy
Even when you are not in REM sleep it is more difficult to take a deep breath while lying down, with or without the above medical conditions, because gravity works against you instead of with you.
Take someone in REM sleep and measure their oxygen levels with a finger pulse oximeter. The levels of someone with normal healthy lungs may drop 3 to 5 %. In lungs that are struggling with any of the above medical conditions the oxygen levels may drop 5 to 10 %, leaving them at oxygen levels that are not heart-friendly for much of the night.
If your oxygen levels drop low enough, the brain will take over and wake you up just enough - usually without you even knowing it - to get you into a lighter stage of sleep where there is no paralysis and you can take a deep breath to restore your oxygen levels to a healthy 90-100% saturation. That is what usually happens in obstructive sleep apnea.
If your oxygen levels dip during REM sleep, low enough to make your heart take notice, but not low enough to cause you to transition to a lighter stage of sleep, you will just naturally cycle back and forth from lowish oxygen levels during REM stages and normal oxygen levels during non-REM sleep. This cycling from lowish to normal to lowish oxygen levels all night, every night, puts an enormous strain on the heart. More so than if your levels just stayed lowish all night.
Over time, your heart can become damaged and develop an irregular rhythm or rate. Or, to help make up for the lack of oxygen, the heart sometimes beats faster, grows larger or causes the body to produce extra red blood cells so that it can use oxygen more efficiently. Unfortunately more red blood cells cause the blood to get thicker, which in turn puts a strain on the heart. All of these "fixes", can lead to heart failure and/or high blood pressure, which in turn makes you more prone to a heart attack or stroke.
Signs & symptoms of heart failure
Excessive daytime sleepiness - EDS
Waking up gasping for breath
Reduced memory, attention and concentration
Puffy ankles, that do not respond effectively to meds
Frequent nocturnal urination (the frequent micro-arousals lead to occasional full awakenings, causing people to think they woke up because they needed to go to the bathroom. Emptying the bladder several times a night confuses the bladder and kidneys so that instead of concentrating and holding the urine as they should at night, they switch to day-mode & then you do need to empty your bladder.)
Is hypoventilation the cause of your heart failure?
Untreated or under-treated asthma often causes night time awakenings that result in excessive daytime sleepiness.
Signs & symptoms of asthma
Waking up at night coughing, gasping, wheezing or short of breath.
Diagnostic tests for asthma
Induced sputum assessment to check for eosinophils
Identify and remove any environmental triggers from the bedroom.
If the asthma is untreated, do a full work up with education and medication as appropriate.
If the asthma is under-treated, add medications to control night-time symptoms.
GERD / REFLUX / HEARTBURN
There is a valve at the esophagus end of the stomach that is supposed to keep the stomach contents as well as the stomach acid in the stomach, thus preventing it from backing up into the esophagus where it can cause heartburn.
Night time heartburn is more common than daytime heartburn and more common as we get older for three reasons. One, the muscles that keep that valve closed are affected by the REM paralysis; two, they become weaker in general as we age and three, when you are lying down, gravity doesn't help keep the acid from backing up the way it does when you are upright. Untreated or under-treated heartburn or "reflux disease" can cause night time awakenings that lead to excessive daytime sleepiness, serious damage to the esophagus or possible permanent lung damage.
Signs & symptoms of heartburn or "GERD"
Daytime burning sensation in the chest.
Night time burning sensation in the chest.
Waking up coughing or choking.
Having an acid taste in your mouth when you wake up.
Waking up with a sore throat.
Night time study using a pH probe in your esophagus
Endoscopy to visualize damage to the esophagus from stomach acid
Do not eat any suspect foods, especially carbonated beverages, fatty (i.e., potato chips and french fries), spicy (i.e., curry, peppers, garlic), or acid (i.e., tomato sauce, orange juice) foods, or alcohol or chocolate within 4 to 6 hours of bedtime.
When you are symptom-free during the night, add foods back in one at a time, every 4 to 5 days so that you can pinpoint the culprit food, which then must be avoided or eliminated. Choosing to eat the culprit food and take antacids is an alternative.
Don't lie down for at least 3 hours after a meal.
Put 6 inch blocks under the head of your bed to raise it up.
NASAL or SINUS ISSUES
Anything that interferes with your ability to breathe well can interfere with your ability to sleep well. A bedroom that is too hot or dry, nasal oxygen or allergies to a pet, stirred up dust, pollen, etc. can cause a nasal or sinus irritation that causes your sleep to be less than restful.
Signs & symptoms
Drink 6 to 8 glasses of water daily, unless otherwise instructed by your doctor.
Use an over-the-counter nasal saline spray to prevent the irritation. Tip your head forward, block off the other nostril and put a couple squirts in each nostril before bed and first thing in the morning. Use it more often, if needed; it's just salt water, like your tears.
Use a room humidifier during the heating season to maintain the humidity level between 30 and 40%.
For allergy sufferers, identify the offending allergen. If outdoor allergens are the culprit, close the windows and use an air conditioner (at least in the bedroom) when your allergens are stirring. If indoor allergens such as dust mites are the issue, wash all your bedding weekly in hot (100 degree) water and encase the mattress and pillow in mite-proof cases. Remove all rugs, carpet, curtains and other such dust trapping surfaces from the bedroom. Sometimes, the use of antihistamines or steroid nasal sprays are helpful until the cause of the allergies can be tracked down and eliminated.
If you have a morning cough and are not a smoker or haven't been one for quite a while, the cough might be due to a nightly post-nasal drip from an allergy.
If you have a runny nose, use the saline spray. If you add the moisture, your nose won't have to add its own