New findings of cortical activity during delta wave sleep sheds further light into memory formation

Scientists at the Center for Interdisciplinary Research in Biology have shown that delta waves emitted while we sleep are not generalized periods of silence during which the cortex rests, as has been described for decades in the scientific literature. Instead, they isolate assemblies of neurons that play an essential role in long-term memory formation. These results were published in journal Science.

While we sleep, the hippocampus reactivates itself spontaneously by generating activity similar to that while we are awake. It sends information to the cortex, which reacts in turn. This exchange is often followed by a period of silence called a ‘delta wave’ then by a rhythmic activity called a ‘sleep spindle’. This is when the cortical circuits reorganize to form stable memories.

However, the role of delta waves in the formation of new memories is still a puzzle: why does a period of silence interrupt the sequence of information exchanges between the hippocampus and the cortex, and the functional reorganization of the cortex?

The authors here looked more closely at what happens during delta waves themselves. They discovered, surprisingly, that the cortex is not entirely silent but that a few neurons remain active and form assemblies, i.e. small, coactive sets that code information. This unexpected observation suggests that the small number of neurons that activate when all the others stay quiet can carry out important calculations while protected from possible disturbances.

And the discoveries from this work go even further! Spontaneous reactivations of the hippocampus determine which cortical neurons remain active during the delta waves and reveal transmission of information between the two cerebral structures. In addition, the assemblies activated during the delta waves are formed of neurons that have participated in learning a spatial memory task during the day. Together these elements suggest that these processes are involved in memory consolidation.

To demonstrate it, in rats the scientists caused artificial delta waves to isolate either neurons associated with reactivations in the hippocampus or random neurons. Result: when the right neurons were isolated, the rats managed to stabilize their memories and succeed at the spatial test the next day.

Cost of sleep deprivation

A lack of sleep among the U.S. working population is costing the economy up to $411 billion a year, which is 2.28 percent of the country’s GDP, a new report finds. According to researchers at the not-for-profit research organisation RAND Europe, part of the RAND Corporation, sleep deprivation leads to a higher mortality risk and lower productivity levels among the workforce, putting a significant damper on a nation’s economy.

Effects of sleep deprivation.
Credit: Mikael Häggström & Wikipedia
A person who sleeps on average less than six hours a night has a 13 percent higher mortality risk than someone sleeping between seven and nine hours, researchers found, while those sleeping between six and seven hours a day have a 7 percent higher mortality risk. Sleeping between seven and nine hours per night is described as the “healthy daily sleep range”.
In total, the U.S. loses just over 1.2 million working days a year due to sleep deprivation among its working population. Productivity losses at work occur through a combination of absenteeism, employees not being at work, and presenteeism, where employees are at work but working at a sub-optimal level.

The study – ‘Why Sleep Matters – The Economic Costs of Insufficient Sleep’- is the first of its kind to quantify the economic losses due to lack of sleep among workers in five different countries – the U.S, UK, Canada, Germany, and Japan. The study uses a large employer-employee dataset and data on sleep duration from the five countries to quantify the predicted economic effects from a lack of sleep among its workforce.

Marco Hafner, a research leader at RAND Europe and the report’s main author, says: “Our study shows that the effects from a lack of sleep are massive. Sleep deprivation not only influences an individual’s health and wellbeing but has a significant impact on a nation’s economy, with lower productivity levels and a higher mortality risk among workers.”

He continues: “Improving individual sleep habits and duration has huge implications, with our research showing that simple changes can make a big difference. For example, if those who sleep under six hours a night increase their sleep to between six and seven hours a night, this could add $226.4 billion to the U.S. economy.”

The U.S. has the biggest financial losses (up to $411 billion, which is 2.28 percent of its GDP) and most working days lost (1.2 million) due to sleep deprivation among its workforce. This was closely followed by Japan (up to $138 billion, which is 2.92 percent of its GDP, and around 600,000 working days lost).

Germany (up to $60 billion, which is 1.56 percent of its GDP, and just over 200,000 working days lost) and the U.K (up to $50 billion, which is 1.86 percent of its GDP, and just over 200,000 working days lost) have similar losses. Canada was the nation with the best sleep outcomes, but still has significant financial and productivity losses (up to $21.4 billion, which is around 1.35 percent of its GDP, and just under 80,000 working days lost).

To improve sleep outcomes, the report outlines a number of recommendations for individuals, employers and public authorities:

Individuals – Set consistent wake-up times; limit the use of electronic items before bedtime; and physical exercise during the day.

Employers – Recognise the importance of sleep and the employer’s role in its promotion; design and build brighter workspaces with facilities for daytime naps; combat workplace psychosocial risks; and discourage the extended use of electronic devices after working hours.

Public authorities – Support health professionals in providing sleep-related help; encourage employers to pay attention to sleep issues; and introduce later school starting times.

Citation: Hafner, Marco, Martin Stepanek, Jirka Taylor, Wendy M. Troxel and Christian van Stolk. Why sleep matters — the economic costs of insufficient sleep: A cross-country comparative analysis. Santa Monica, CA: RAND Corporation, 2016. Link
DOI: 10.7249/RR1791
Adapted from press release by RAND Corporation.

Parental sleep behavior affects children’s sleep

 A new study indicates that children’s sleep duration may be influenced by parental sleep duration and confidence, which suggests that efforts to address insufficient sleep among children may require family-based interventions.

Results of a parental survey show that higher parent confidence in the ability to help children get enough sleep was significantly associated with an increased child sleep duration of 0.67 hours per day, after controlling for potential confounders such as child age, gender, race/ethnicity, and parent education. Overall, 57 percent of parents reported feeling “very” or “extremely” confident that they could help their child get enough sleep. The study also found that child sleep duration was 0.09 hours per day longer for each 1-hour increase in parent sleep duration. Study results are published Journal of Clinical Sleep Medicine.

“Our study suggests that educating parents about their own sleep health and promoting increased confidence in their ability to help their children get enough sleep are potential areas of intervention to increase child sleep duration, either through formal programs or in a pediatricians office,” said lead author Corinna Rea, MD, instructor in pediatrics at Harvard Medical School and attending physician at Boston Children’s Hospital.

To promote optimal health, the American Academy of Sleep Medicine recommends that children between the ages of 6 and 12 years should sleep 9 to 12 hours per 24 hours on a regular basis to promote optimal health. Regularly sleeping fewer than the number of recommended hours is associated with attention, behavior, and learning problems, and it increases health and safety risks. The study also evaluated the relationship between child sleep duration and other parent behaviors and practices, including screen time, physical activity, and limits placed on child TV viewing. Surprisingly, after adjustment for demographic characteristics, these behaviors were not significantly associated with child sleep duration. “Our results also may suggest that individual parent behaviors do not reflect a ‘family lifestyle,’ but rather that parental sleep is directly linked to child sleep irrespective of others behaviors,” explained Rea.

The study involved 790 parents with a mean age of 41 years. Their children, who were between the ages of 6 and 12 years, were participating in a randomized controlled obesity trial. Trained research assistants administered a survey to parents over the phone. About 92 percent of respondents were mothers. Average daily sleep duration was 6.9 hours for parents and 9.2 hours for children.

According to the authors, the cross-sectional design of the study did not allow for an examination of causality. However, the authors noted that there are several potential mediators for the association between parent and child sleep duration. For example, parents may influence child sleep duration by serving as role models, encouraging and supporting their child’s healthy choices, or establishing a family bedtime.

Citation: Rea, C. J., R. L. Smith, and E. M. Taveras. “Associations of Parent Health Behaviors and Parenting Practices With Sleep Duration in Overweight and Obese Children.” Journal of clinical sleep medicine: JCSM: official publication of the American Academy of Sleep Medicine (2016).
DOI: 10.5664/jcsm.6274
Research funding: American Recovery and Reinvestment Act, National Institute of Diabetes and Digestive and Kidney Disease / National Institutes of Health.
Adapted from press release by American Academy of Sleep Medicine.