What do you usually do before bed? Relaxing activities such as reading, drinking a cup of herbal tea or practicing yoga are generally recommended to reduce stress and help you fall asleep faster, resulting in better sleep. In contrast, using digital technology and drinking alcohol can impair sleep quality.
Although insomnia is the most common sleep disorder, little is known about the role of genetics in its origin and development. Between 6% and 10% of people suffer from chronic insomnia, while about 25% experience occasional sleep difficulties. Insomnia is associated with several negative effects, such as fatigue, irritability, and difficulties with concentration and memory. Given its high prevalence and impact on public health, better understanding the genetic basis of insomnia could lead to advances in prevention and treatment, benefiting a large proportion of the population.
Several genome-wide association studies (GWAS) have been conducted on sleep phenotypes in humans. However, to further understand the genetics of sleep regulation and insomnia, studies that explore a broader range of insomnia phenotypes and have sufficiently large sample sizes to detect small effects are needed. In this study, a GWAS of 6 sleep phenotypes (bedtime, sleep latency, sleep quality, sleep depth, sleep duration, and an insomnia factor score) was performed using questionnaire responses from 2,323 individuals from the Australian Twin Registry. To assess habitual sleep timing, participants were asked about the time they usually attempted to fall asleep after bedtime. Individuals who attempted to fall asleep between 3:00 AM and 6:00 PM were excluded from the analysis (n = 23) as they could be night workers or have delayed sleep phase syndrome. Over 2 million common genome-wide polymorphisms were tested. Although no SNP reached genome-wide significance, several associations with plausible candidate genes were identified. One example is rs722258 at LOC107984215, where the T allele showed an additive effect indicating an earlier sleep schedule.
Additionally, two SNPs located in the L3MBTL4 and EBF3 genes were also associated with sleep timing. L3MBTL4, located on chromosome 18, has a poorly understood function, as does LOC107984215. EBF3, located on chromosome 10, is expressed in the brain. Interestingly, no circadian gene related to the human internal biological clock showed strong evidence of association with sleep timing in this analysis. Another SNP near the NPS gene also showed an association with sleep timing. NPS is a relevant gene because it encodes Neuropeptide S, a molecule associated with arousal, anxiety and sleep apnea.
The study has some limitations, such as the lack of replication for some associations, since replication was only performed for sleep latency. Confirmation of the associations in independent samples is necessary to validate the results and prevent them from being mere chance events.
