Now, a new study from India suggests that extra hours in bed don't mean much in terms of benefits, if there isn't also a corresponding increase in good-quality slumber.
The research, which involved 452 low-income workers across the course of a month in Chennai, also found that an afternoon nap was more beneficial than an additional hour of sleep overnight – at least in the study participants who had very disrupted night-time sleep.
Measurements were taken using actigraphs: small, wearable motion sensors that are able to monitor sleep cycles, which are becoming more popular in scientific research. Actigraphs can take readings as people sleep in their own homes, without the need for any extra equipment or complicated configuration.
By providing information and encouragement, along with improvements to home sleep environments, the researchers were able to get the workers sleeping for almost an extra half-hour each night, on average – but the expected health benefits didn't follow.
"To our surprise, these night-sleep interventions had no positive effects whatsoever on any of the outcomes we measured," says economist Frank Schilbach from the Massachusetts Institute of Technology (MIT).
Those outcomes included cognition, productivity, decision-making, and well-being. The number of hours worked on average went down as well – perhaps because more time in bed meant less time to work. The volunteers were engaged in a data-entry job designed solely for the study, where their attention and output could be measured.
It's important to point out the other conditions of the study too. Those involved were averaging just 5.5 hours of sleep from eight hours in bed per night, before the researchers were able to push this up.
By the end of the month of experiments, the workers were spending an extra 38 minutes per night in bed, which equated to an extra 27 minutes per night of actual sleep.
This low sleep efficiency would seem to prevent the deeper, more restorative type of sleep that can be so advantageous to overall health. Plenty of previous research has highlighted the consequences of not getting enough quality shut-eye each night, including an increased risk of dementia.
In their everyday lives, the volunteers included in the study tended to wake up about 31 times each night on average, the researchers point out. In other words, their existing sleep quality is comparable to someone in a wealthy country who has issues with insomnia or sleep apnea.
"In Chennai, you can see people sleeping on their rickshaws," says Schilbach. "Often, there are four or five people sleeping in the same room where it's loud and noisy, you see people sleep in between road segments next to a highway.
"It's incredibly hot even at night, and there are lots of mosquitos. Essentially, in Chennai, you can find any potential irritant or adverse sleep factor."
Half of the participants were also encouraged to nap for half an hour during the day, and this did lead to several positive results: improvements in productivity, cognitive function, and psychological well-being were noted.
Once again, though, time working dropped – the participants didn't want to or weren't able to make up the time they spent napping, even though they were getting more work done when they were actually on the job.
The researchers say we need more studies like this in developing nations, rather than just in sleep labs in wealthier countries – and also caution against sleep studies being applied too broadly across different nations and communities.
Future studies could concentrate on sleep quality rather than sleep duration, the research team suggests, while psychological factors – such as the stress and worry often faced by families on lower incomes – could also be taken into account.
"There's not a lot of work studying people's sleep in their everyday lives," says Schilbach. "And I really hope people will study sleep more in developing countries and poor countries, focusing on outcomes that people value."
The research has been published in The Quarterly Journal of Economics.