Brain waves are rhythmic patterns of electrical activity that change in frequency and amplitude according to what the brain is currently doing. These brain waves are typically organised into different frequency bands, ranging from slow delta waves with frequencies less than 2Hz to fast gamma waves with frequencies greater than 40Hz. Each of these frequency bands are thought to be associated with different cognitive and behavioural processes.
There are distinct patterns of brain waves that occur during the different stages of sleep and these sleep brain waves may be pathologically altered by sleep disorders such as insomnia. For example, there is a stage of deep sleep called slow-wave sleep that is characterized by low frequency brain waves commonly called slow-waves. These slow-waves are thought to play an important role in reinforcing new memories and motor skills that were learned during the day, which may explain why the proportion of slow-wave sleep is typically greatest in childhood (when there is a lot of new information and motor skills that the brain needs to learn) and gradually decreases with age.
Over the last 20 years, researchers have used a type of non-invasive brain stimulation called transcranial alternating current stimulation (or tACS) to further investigate the functional roles of different brain waves in cognition and behaviour. This involves the application of a weak electrical current to the scalp that matches the frequency of a particular brain wave, with the goal of enhancing the activity of that brain wave. tACS research actually began by targeting slow-waves during sleep and there have been multiple studies demonstrating that slow-wave tACS can increase subsequent slow-wave activity in the brain, resulting in significant improvements in sleep-dependent memory and motor learning.
Recently researchers have started using tACS at different sleep frequencies to try and correct sleep brain waves that have been altered by insomnia. Whilst this research has demonstrated some subjective improvements in sleep quality and insomnia severity scores, the findings have been somewhat hindered by the limited number of studies that have been performed thus far, which is further exacerbated by the fact that there is considerable variability between these studies. Therefore, whilst tACS may prove to be a useful option for the treatment of sleep disorders in the future, there is still a lot more research that needs to be done before it sees widespread clinical use.
Sources:
Herrmann, C. S., Strüber, D., Helfrich, R. F., & Engel, A. K. (2016). EEG oscillations: From correlation to causality. International journal of psychophysiology: official journal of the International Organization of Psychophysiology, 103, 12–21. https://doi.org/10.1016/j.ijpsycho.2015.02.003
Marshall, L., Helgadóttir, H., Mölle, M., & Born, J. (2006). Boosting slow oscillations during sleep potentiates memory. Nature, 444(7119), 610–613. https://doi.org/10.1038/nature05278
Geffen, A., Bland, N., & Sale, M. V. (2021). Effects of Slow Oscillatory Transcranial Alternating Current Stimulation on Motor Cortical Excitability Assessed by Transcranial Magnetic Stimulation. Frontiers in human neuroscience, 15, 726604. https://doi.org/10.3389/fnhum.2021.726604
Khan, B.W., Khattak, M.H., Khan, S. et al. (2025). Efficacy of transcranial alternating current stimulation for insomnia: a GRADE-assessed systematic review and meta-analysis of randomized controlled trials. Sleep Science Practice 9, 39. https://doi.org/10.1186/s41606-025-00163-6
