Shining a light on sleep disturbance

Dominic Acquista dominic.acquista@photonics.com

A good night’s sleep can depend on many factors. Physical stressors, such as loud noises or extreme light, might interrupt sleep patterns temporarily, but underlying medical conditions, specifically stress-related diseases, can have long-term health implications. While it sounds counterintuitive, clinical evidence suggests that to alter sleep patterns for the better, it is a good idea to turn the light on. Researchers recently explained why bright lights could be just what the doctor ordered for those with chronic sleep issues.

Using mice as their test subjects, a group of researchers from Jinan University in China set out to understand the mechanics behind why light treatment seems to be effective in affecting sleep patterns. They focused specifically on the lateral habenula (LHb) of the epithalamus, which encodes stressful signals and sends them directly to regions of the brain that are associated with sleep regulation. Depending on how neural activity changes in the LHb, it can worsen the way stress affects one’s sleep. However, light signals transmitted by visual circuits to the LHb may help to regulate it.

Researchers set out to understand how bright-light treatments could induce healthy sleeping patterns in mice. Courtesy of iStock.com/socris79.

After exposing the mice to stressful stimuli for a period of two weeks to introduce chronic stress, researchers conducted a series of chemogenetic and optogenetic tests to understand their responses to light. This included using fiber photometry and obtaining EEG spectrograms to understand how neurons in the LHb responded to the external stimuli. Quite predictively, the stimuli increased depressive-like symptoms, restless sleep, and agitation in the mice without affecting their motor function. But they also found that it had a significant effect on their sleep- and wake-states. Not only did the mice show a decrease in wakefulness during both the day and night, but the researchers found an increased duration of non-rapid eye movement (NREM) while they were asleep.

Both stages of sleep, rapid eye movement (REM) and NREM, are important to any good night’s rest, and ideally, they exist in balance. Though research suggested that REM sleep was not significantly affected by chronic stress, it does suggest an increase in the number of transitions between wakefulness and NREM sleep, thus likely causing tossing and turning in both a literal and figurative sense. This turbulence then affects overall sleep duration.

The team found that this phenomenon is caused by the light-based activation of rostromedial tegmental neurons in the LHb, which were the only subgroup of neurons, among others that were studied, that significantly increased NREM sleep in both the day and night. This discovery proved to be significant because when the researchers conducted experiments in which they introduced neuron inhibitors in the mice to block the flow of rostromedial tegmental neurons, they found that it had the exact same effect as bright-light treatment.

Now that the researchers know how bright light can alter and regulate the NREM stages of sleep, they concluded that the results might deepen additional researchers’ and clinicians’ understandings of the structure and function of light on the human psyche. Indeed, the switch has been turned on with a working theory of how bright light may help alleviate a host of stress-related disorders — and they have no plans to sleep on it.