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  • Writer's picturePetra

Understanding Circadian Rhythm: Impacts on Sleep and Mood, Plus Light Therapy Interventions

This is a longer blog post than normal, but is on an important topic that can affect many aspects of health and wellbeing - our internal body clock, or what is known as the 'circadian rhythm.' This post explores what circadian rhythm is, and its implications for sleep, mood, and the potential treatment options it presents.


What is the Circadian Rhythm?

In essence, the circadian rhythm is our body's internal clock, orchestrating various physiological functions over a roughly 24-hour cycle. It's our internal metronome, keeping time and rhythm for various processes, from our sleep-wake cycle to hormone secretion, and even mood regulation.


The circadian rhythm is naturally aligned with the day-night cycle, and this alignment is crucial for our overall wellbeing. However, when misaligned – due to factors like shift work, travel across time zones, irregular sleep habits, or a person's own genetic predisposition – it can contribute to health issues, including sleep disorders and mood disturbances. I often see circadian rhythm difficulties in my clients with ADHD, primarily a delayed circadian rhythm.


Circadian Rhythm and Sleep

One of the most apparent manifestations of our circadian rhythm is our sleep-wake cycle. Under ideal circumstances, our circadian rhythm aligns with external cues, such as light and darkness, to guide us into sleep and wakefulness at appropriate times.


When darkness descends, our bodies increase the production of melatonin, a hormone that makes us feel drowsy and signals it's time for sleep. As morning light emerges, melatonin production decreases, cueing our bodies to wake up. When this process gets disrupted, it can lead to trouble falling asleep, staying asleep, or feeling fully rested upon waking – common symptoms of insomnia and other sleep disorders. I have a more detailed blog post about the circadian rhythm sleep disorder delayed sleep phase syndrome, including using timed light and dark exposure, and sometimes melatonin as an evidence-based treatment.


Circadian Rhythm, Mood, and Depression

Our circadian rhythm doesn't just regulate sleep. It also has a substantial influence on our mood. Disruptions to the circadian rhythm can contribute to mood disorders, including depression. This connection becomes more apparent in Seasonal Affective Disorder (SAD), a form of depression that typically occurs during the shorter, darker days of winter. This condition is linked to a mismatch between our internal clock and the external environment and is exacerbated when opportunities for natural light exposure are reduced.


Even in non-seasonal depression, circadian rhythm disruption can play a part. Some individuals with depression experience disturbed sleep patterns, altered hormone secretion, and changes in other physiological processes that a healthy circadian rhythm typically regulates.


Light Therapy and Circadian Rhythm

Light therapy, also known as phototherapy, can be an effective intervention to strengthen and realign circadian rhythm. It involves exposure of the eyes to specific wavelengths of light for a prescribed amount of time, usually in the morning. The light acts as a powerful cue for our circadian rhythm, which is governed by a part of the brain called the suprachiasmatic nucleus of the hypothalamus, helping entrain the sleep-wake cycle and improving sleep quality.


The circadian rhythm responds most powerfully to blue light, particularly in the wavelength range of approximately 480 nanometers (nm). This wavelength presents as sky blue to our eyes. While this blog post focuses on light exposure, avoiding blue light in the evening for the few hours before bed - which includes white light and particularly LED lighting including from computer and phone screens, is the complement to light exposure in the morning, and will aid with increasing melatonin production at sleep time.


Timed light exposure can be effective at shifting the circadian rhythm to acclimate to a new time zone and to reduce the vicissitudes of jetlag. There are online planners that can offer advice on how to achieve this.


Light Therapy for Seasonal and Non-Seasonal Depression

For individuals with conditions like SAD or other types of depression, light therapy can be beneficial. In fact, light therapy is the first-line treatment for SAD, which is great, since it is medication-free, can start to take effect within several days, and is substantially cheaper than a course of therapy.


By targeting the underlying circadian disruption that can occur in these conditions, light therapy can offer a complementary approach to traditional treatments such as medication and psychotherapy. I suggest light therapy for my clients when a seasonal pattern of depression is evident, but I should probably also be recommending it more widely for depression in general, and perhaps also for premenstrual dysphoric disorder (I have found several studies that show that light therapy can significantly reduce depression in the luteal phase of the menstrual cycle for people with premenstrual dysphoric disorder).


Light therapy for stabilising circadian rhythm and for treating depression is suggested at ~30 minutes per day, ideally in the first half of the morning. The cheapest way to do this is to go for a walk in the morning, without wearing sunglasses or a hat with a brim. Light needs to land on the eyes. Interior lighting is not sufficient to reset circadian rhythm in the morning, and sitting next to a window is unlikely to be enough either. For people with depression, a 30-minute walk in the morning can also be helpful for mood, and walking with someone else can also add a helpful social component.


But, many people aren't able to go for a 30-minute walk each morning, and in winter when the sun is rising close to 7:30 am, a working person might leave the house and return home in darkness.


Note: light therapy can promote hypomania or mania in those with bipolar disorder. If you have bipolar disorder please use light therapy under the supervision of a suitably qualified health practitioner.


Light Therapy Products

Before the advent of powerful light-emitting diodes (LEDs) and lithium-ion rechargeable batteries, light therapy was delivered using large light boxes that a person had to sit in front of for 30 minutes in the morning. Thankfully, nowadays there are a number of lightweight glasses or visors available for purchase. The below light therapy products all provide at or above the minimum of 10,000 lux of light required to strengthen circadian rhythm in ~30 minutes.


These emit blue light from the top of the frame over the eye area. They have a 30-minute timer that turns the glasses off at the end of treatment. They come with blue and amber-tinted exchangeable lenses. Blue lenses are for morning use, while amber lenses are to filter out blue light in the evening, which aids in melatonin secretion.


These are designed to fit over glasses and emit light from above. The light is white and reflected off a holographic sticker which splits it into a spectrum with the blue part falling on the eyes. There are three light levels that are correspondingly timed at 60, 30, and 20 minutes (20 being the brightest).


These use blue-green light and can fit over glasses. Light is emitted from below the eye, which can be less comfortable than light emitted from above as the other devices use.



AYO is a visor that can fit over glasses and that emits blue-turquoise light from above in a 20-minute timed session duration. It comes with an optional app subscription that does all sorts of customisation, but you can also just buy the visor without this added service.




All of these products can be purchased off Amazon.com and sometimes go on sale. I have personally used the Re-Timer, Propeaq, and the Luminette 3 and have a Propeaq pair in my office to show clients. I do not sell these products, or earn money through affiliate programs for the sale of these products.


  • While I have simplified it here, circadian rhythm is a complex process that is well understood in plants and animals (humans included) through physiological research. If you want to deep-dive into this area, the Wikipedia article on the subject is a good starting point.

  • For an excellent summary of current understanding of circadian rhythm and how it affects adults with ADHD, including looking at both sleep and seasonal depression, please see the one-hour webinar How Seasonal Affective Disorder Uniquely Affects People with ADHD presented by psychiatrist and ADHD researcher Dr J.J. Sandra Kooij as part of the ADHD Experts webinar series.






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