Med thumb legs holding up earth solar system thumb

Sharks nap in squads

Sharks don't sleep in the way we understand the vital neurobiological activity, but they do fit in restful periods throughout the day. Some larger, oceanic shark breeds practice "yo-yo diving," in which they swim to the water's surface for rest, descend back into the sea and then repeat this back-and-forth behavior for a few minutes at a time. But there are other shark breeds that huddle together on the ocean floor to get their pseudo-shuteye. And this type of shark sleep was caught on camera by scuba divers off the coast of Mexico. Check out this video of 20 or so whitetip reef sharks dozing in a pile. (Insofar as sharks can be cute, it's cute.) [National Geographic]

The United States of sleep inequality, vol. XII

In an effort to make sense of racial disparities in cardiovascular health, a team of researchers used actigraphy (e.g., fitbit) to monitor the sleep of 426 white and black men and women. Based on seven nights of data, researchers found that black participants got about 40 minutes less sleep per night than white participants. Black participants also exhibited less efficient sleep, meaning they woke up more during the night and took longer to fall asleep. The results jibe with other analyses of sleep and race. The study didn't go so far as to say what's causing racial differences in sleep duration and quality. But one of the study co-authors speculated that neighborhood crime or economic stress may be keeping people up.

Researchers also measured physiological markers of cardiovascular disease, such as blood pressure and waist circumference, and found a connection between poor health outcomes and sleep quality — but mainly for black women. Again, researchers don't know exactly why the findings turned out the way they did, but here's one researcher's guess: "Being a black female in the United States is inherently more stressful than being just a female or being just black." [The Atlantic]

Oversleepers, beware of 'mares  

More often than not, an "unhealthy amount of sleep" means "too little sleep." But it works the other way around, too. Getting more than 9 or 10 hours of sleep a night (for adults) has been linked to a number of health snafus, including increased diabetes risk and obesity. And, courtesy of a new study, here's one more: frequent nightmares. The study, which involved 846 people, was one of the largest-ever explorations of nightmares in the general population. [New Scientist]

Undersleepers, watch your waistlines

People who reported logging less than seven hours of sleep a night, in one UK study, had higher BMIs and larger waistlines than those who got their eight hours. What accounts for the extra pounds? Well, one study co-author implicated weight gain from overeating. “Some of it is that when people are sleep-deprived, they tend to go for high calorie, fatty, good tasting foods." The findings were based on sleep records and food diaries from more than 1,600 participants, as well as blood samples from roughly half of participants. [Reuters

Good sleepers watch their stories one episode at a time 

The ultimate modern-day indulgence is, without a doubt, ordering Seamless + bingeing five (or 10) episodes of a buzzy new show. But, sadly, binge-watching may be a recipe for poor sleep. A new study from American and Belgian researchers links sleep problems and frequent binge-sessions. No such link emerged for watching TV in a non-bingeing manner. Why would binge-watching, specifically, set someone up for rocky rest? Well, we don't know for sure, but here's what researchers think: Because binge-watching is such an immersive, mentally stimulating experience, it puts us in a state of heightened arousal that interferes with our ability to fall (and stay) asleep. In other words? Diversify your TV shows to avoid getting too invested in any one fictional universe. [Van Winkle's]

Today in sad and confusing phenomena: the nightmare-suicide link

A doctor recalls treating a suicidal teen and failing to inquire about one potentially relevant topic: the patient's dreams. Because, even for people who don't have PTSD, nightmares have emerged as a factor for suicide. The relationship between the two phenomena, however, isn't well-understood and hasn't born out in every study. In one case, insomnia, but not nightmares, predicted suicide risk. Part of the difficulty in figuring out what's going on lies in the fact that we don't have a firm grasp on nightmares in the first place. Some researchers believe they function like dress rehearsals for IRL threatening encounters, but that's really just a theory. [Science of Us]