Dear Doctors: I understand how my fitness tracker counts my steps and measures my heart rate, but supposedly it can say how well I sleep. How can it do that, and is that sort of data even accurate?

Dear Reader: We’ve seen a boom in the popularity of health tracking devices in recent years. A poll conducted last year found that 1 in 3 Americans has tried out some form of health tech, and that close to 20% of us now use a digital fitness tracking device each day. Depending on the specific device, wearables can count the number of steps we take each day, log our heart rate, analyze stress, count how many flights of stairs we have climbed, pinpoint the amount of time we’ve spent being active, estimate how many calories we’ve burned, measure distance traveled or use UV sensors to measure sun exposure. And, yes, a growing number of devices now follow us into the bedroom to monitor and analyze our sleep.

Many wearables, like the one you use, include a sleep mode. Again, depending on the device, they may offer information about how long you sleep, or about the quality of that sleep. They do this using sensors known as accelerometers, which can detect not only when someone is moving but also the speed and direction of that motion. This data, along with the heart rate information that the device collects, is extrapolated into sleep analysis. Some trackers even claim to be able to quantify how much time has been spent in the three major phases of sleep -- light sleep, deep sleep and REM sleep.

It’s important to note that for accurate sleep analysis, you need an overnight stay at a sleep lab. There, electrodes are placed on the face, scalp, chest and limbs to digitally collect and record a range of activities in both the brain and the body. The information that goes into analyzing your sleep includes breathing, physical motion, muscle tone, eye movement, heart rate and rhythms, and brain wave activity. Fitness trackers, by contrast, are limited to the motion and heart rate data that they are able to collect, which is then parsed by an algorithm. Sleep experts are concerned that someone with a sleep disorder who relies on the data from a fitness tracker may inadvertently be reassured that all is well. The flip side is that someone with no sleep problems at all may become worried about data that isn’t necessarily accurate.

The bottom line is that a fitness tracker with a sleep mode can be good at letting you know how long you were motionless, how often you were restless and how many times you got up during the night. However, it’s not information that reliably translates into sleep quantity or sleep quality. Anyone who thinks they may have a sleep disorder, or who wants to improve the quality of their sleep, should first check with their health care provider. Then, if it’s appropriate, a night in a sleep lab may be the next step.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10880 Wilshire Blvd., Suite 1450, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)

Hello again, dear readers! You’ve kept our digital and postal mailboxes even more full than usual with your questions, thoughts and (thank you!) kind words. Not surprisingly, much of the discussion focused on COVID-19. To help with the volume of mail, we’ll have a separate column with your virus-related questions next week. And now, onward to the more general discussion.

-- In a column about the search for a saliva test to diagnose Parkinson’s disease, we mentioned the lack of a specific diagnostic test for the condition at this time. A reader from Oklahoma, whose wife lives with Parkinson’s disease, wondered why a test known as a DaTscan wasn’t mentioned. “It is considered reliable enough that it is often required at the start of treatment testing to prove a patient has PD,” he wrote.

DaTscan uses a radioactive isotope to assess the health of the dopamine system in the brain. It’s the loss of dopamine transporters (DaT) that leads to the neurological symptoms of Parkinson’s disease. However, the scan will return an abnormal result in any neurological condition that involves dopamine transporters. It’s used in conjunction with a neurological and physical exam to arrive at a final diagnosis, but in and of itself does not return a definitive PD diagnosis.

-- A reader from Spokane who suffers from chronic foot pain requested a copy of our column about a condition known as Morton’s neuroma. Although we can’t mail you a copy, we can provide you with the link. We hope it helps. uexpress.com/ask-the-doctors/2019/12/9/having-a-mortons-neuroma-is-a

-- In that same vein, a reader from North Carolina is disappointed that her newspaper occasionally abridges our columns. The good news is that you can find the entire (and uncut) Ask the Doctors archive at uexpress.com/ask-the-doctors.

-- A reader whose 76-year-old mother has just received a diagnosis of colon cancer wonders where to learn about clinical trials looking into the disease. An excellent resource is The National Cancer Institute, at cancer.gov. Enter the words “colon cancer clinical trial” in the search bar, and the top three results will give you a wealth of information.

-- To the reader who asked what RNA and DNA stand for, the answer is ribonucleic acid and deoxyribonucleic acid, respectively. These are the names of the sugars that serve as the backbones of the two molecules.

-- A column about hypertension led a reader to ask us to highlight something known as “white coat syndrome.” This is a condition in which someone’s blood pressure spikes in a clinical setting but is otherwise normal. Although there is no surefire cure or mitigation for this, it’s important information for your health care provider to have. They may ask you to wear a portable blood pressure monitor for 24 hours to get an accurate reading.

We’ll wrap up with another thank-you, this time to the sharp-eyed readers who pointed out an error regarding dietary guidelines for daily limits on sodium. You’re correct that the unit of measurement should have been 2,300 milligrams, and not micrograms (sigh), as we wrote.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10880 Wilshire Blvd., Suite 1450, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctors: What can you tell me about Kawasaki disease? I read that children who had COVID-19 are getting it. But I thought children were supposedly safe from the virus. Why has that changed?

Dear Reader: You’ve broached two important topics, and we’ll take them in reverse order. As we’ve mentioned here before, every bit of information about the novel coronavirus, which we all first heard about in the final days of 2019, has been learned in real time. No one had ever seen this specific virus before, so everything about it was a mystery. This included its genetic and physical makeup, how it’s transmitted, who it affects, how it enters the body and how the body responds once an infection is launched. Our understanding of the virus and COVID-19, which is the disease that it causes, continues to evolve on a daily, and sometimes hourly, basis.

You’re correct that at the start of the pandemic, COVID-19 was known primarily as a viral pneumonia. It’s also true that doctors saw significantly fewer cases in children than among adults. People 18 and younger have accounted for only 2% of confirmed cases of COVID-19, and they often experience mild symptoms.

However, as doctors continue to treat patients, the knowledge base about the disease and its many symptoms expands. Now, emerging data points to the novel coronavirus as the cause of a potentially dangerous inflammatory condition in children. Referred to in the U.S. as multisystem inflammatory syndrome in children, or MIS-C, it involves inflammation in the heart, blood vessels, eyes and skin. Although it’s similar to Kawasaki disease, a rare inflammatory illness often seen in infants and young children, it’s not the same thing. Kawasaki disease is a condition in which high levels of inflammation affect the blood vessels that serve the heart. Untreated, it can result in damage that is life-threatening.

Symptoms of MIS-C include fever, swollen lymph nodes in the neck, a skin rash, pink eye, redness of the lips and tongue, intestinal distress, inflammation leading to poor organ function that can include the kidneys or heart, inflammation of the blood vessels and joint pain. The two most common symptoms of COVID-19 in adults, which are shortness of breath and a dry, hacking cough, often don’t show up in children. Doctors in 19 states have reported cases of the new syndrome, including hard-hit New York, New Jersey and California. The condition is also being seen in Europe and the United Kingdom.

Researchers don’t yet know what triggers the inflammatory syndrome, which is not caused directly by the novel coronavirus. However, many of the affected children are testing positive for COVID-19 antibodies. This suggests a link between MIS-C and the novel coronavirus. Some researchers suspect the syndrome is a delayed response by the immune system.

So far, the syndrome is rare, and most children are not seriously affected. However, new cases continue to emerge. This has prompted the Centers for Disease Control and Prevention to prepare a national alert with detailed information on symptoms, so that the syndrome can be quickly identified and treated.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10880 Wilshire Blvd., Suite 1450, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)