People aren't getting enough sleep, Apple Watch data shows

Daylight saving time starts Sunday, and everyone will lose an hour of sleep -- which most people can't afford to lose, according to new data from Apple and the Brigham and Women's Hospital.

As part of the Apple Heart & Movement Study, researchers reviewed data from over 2.9 million nights of sleep. They found that among over 42,000 people who have Apple Watches and participated in the study, only 31% of people are meeting the minimum recommended 7 hours of sleep per night. They published the data, which has not been peer-reviewed, on the study's website.

The American Heart Association Life's Essential 8 measures recommends an average of 7 to 9 hours of sleep per night. Poor sleep may put you at risk for cardiovascular disease, cognitive decline and dementia, depression, obesity, and higher blood pressure, blood sugar, and cholesterol levels.

The study findings reinforce what sleep experts know about sleep trends: when it comes to our nightly sleep goals, "the vast majority of us are falling short," Dr. Rebecca Robbins, a researcher of sleep medicine at Harvard Medical School, tells ABC News.

Weekend sleep trends, the social jetlag

For this analysis, researchers collected data from the Apple Heart & Movement Study, an ongoing study anyone with an Apple Watch and iPhone can sign up for using the Apple Research app. Participation is 100% opt-in. The data can only capture people who own iPhones and Apple Watches, and was sponsored by Apple.

The study gives the team access to a huge swath of data. "We have the fortunate results of technology to be able to look at sleep in over 40,000 individuals," Dr. Calum MacRae, cardiologist at Brigham and Women's Hospital and lead investigator of the study, told ABC News.

Along with sleep duration, the study also reported differences in sleep start time on weekdays versus weekends. On weekdays, people went to bed before midnight about 66% of the time. That number drops to about 57% on weekends.

Sleep experts refer to this phenomenon as the social jetlag, which describes the delay in our internal body clock that happens when you stay up late for social reasons.

The consequence? We "cheat a bit on our sleep schedules and make up this by flipping it on the weekends," said Robbins.

Robbins suggested ways to manage social jetlag is to try to avoid having late nights on both nights of the weekend. And, if you do stay out late, try to wake up as close to you normal time on Saturday or Sunday morning.

If you are tired in the afternoon, you can make up lost sleep with a nap. The Center for Disease Control and Prevention says either a power nap of about 15 to 30 minutes or a longer nap up to 90 minutes can increase alertness. This can bring relief for "anyone who has been burning the candle at both ends during the workweek, too," Robbins said.

Sleep numbers by the states

Across all states, the study found that fewer than 40% of residents met the AHA's recommended sleep duration of 7 to 9 hours per night.

Washington, South Dakota and Idaho had the highest proportion of people meeting the AHA's nightly sleep recommendation. Hawaii, Mississippi and West Virginia had the lowest proportion of people meeting the AHA's nightly sleep recommendation.

The study researchers say these differences could be attributed to cultural or demographic differences. Factors like age, employment status and job type all contribute to differences in sleep patterns. Time zones also play a role, as they "can affect sunlight hours and play a part in the total amount of sleep an individual will obtain," said MacRae.

Tips for better sleep health

A major tip for a better night's sleep is to keep a consistent sleep schedule. The CDC recommends going to bed at the same time each night and getting up at the same time each morning, including on weekends.

Keeping consistent with a regular bedtime will "help your internal clock know when it's supposed to be tired," said Robbins.

Other strategies the CDC recommends are avoiding large meals, caffeine and alcohol before bedtime, and removing electronic devices such as TVs, computers and smartphones from the bedroom. Additionally, making sure your bedroom is quiet, dark, relaxing and a comfortable temperature are ways to optimize sleep health.

"We're not quite at our sleep goal, but do your best to make the time to sleep," Robbins said.

Kimberly Loo, MD is an internal medicine resident at New York-Presbyterian/Weill Cornell Medical Center and a contributor to the ABC News Medical Unit.