Light in the blue wavelengths sets our circadian rhythm. Great when we get sunshine in the morning, but artificial light at night is messing us up!
Basic overview: Blue light (~480nm) hits our retinas and suppresses melatonin production. Prior to electric lights – and especially TV, computers, smartphones – we only got blue light during the day when the sun is up. Melatonin is thought of as the 'sleep hormone' because levels rise at night, but it also acts as a signaling molecule for our core circadian clock as well as acting in our immune system and as an antioxidant. (More in-depth explanation.)
Sleep disruption and ADHD symptoms go hand in hand. But the idea that circadian rhythm disruption is causing, or at least contributing to, ADHD is a newer topic for researchers.
Keep in mind as I explain the research on the role of circadian rhythms in ADHD, it is probably not quite as simple as: light at night = ADHD. This is a complex topic with more than one variable at play.
ADHD has core symptoms of inattentiveness, impulsivity, and restlessness. For some, this leads to problems in social or academic settings. Dopamine, a neurotransmitter, is thought to be at the root of the problem for many with ADHD, and medications such as Concerta target this pathway.
ADHD and Circadian Disruption - Genetic Basis:
One way that researchers search for causes of a condition is to look for genetic variants that are thought to cause the condition. A lot of ADHD research has been focused on neurotransmitters, and genetic variants in the dopamine, serotonin, and norepinephrine systems seem to add a little bit to the risk for ADHD.
Dopamine synthesis and secretion interacts with circadian rhythm genes and follows a cycle over the course of a 24-hour day. Specifically, it varies and peaks according to the rhythm of a core circadian clock protein, PER2. Dopamine has been shown to rise and fall in an opposite pattern to melatonin, and it also is affected by the melanopsin photoreceptor in the retina that sets our body's core circadian clock.
Another core circadian gene, aptly named CLOCK, has also been tied to an increased risk for ADHD. A fairly common genetic variant in CLOCK was shown to be significantly associated with increased attention problems and hyperactivity scores when compared to people without the variant. In fact, CLOCK variant carriers had more than double the scores of those without the variant.
Research studies showing circadian disruption and ADHD:
A 2018 study (Nighttime media use in adolescents with ADHD: links to sleep problems and internalizing symptoms) found that the average nighttime media usage in teens with ADHD (in the study) was 5.31 hour - after 9pm! About three-quarters of the teens were getting less than 8 hours of sleep. This study of ADHD teens found that "nighttime media use was associated with shorter sleep duration and increased sleep problems across both adolescent and parent report. Media use was also associated with greater adolescent-reported anxiety and depression..."
Seasonal affective disorder, which is a circadian disruption disorder that leads to depressive symptoms, is also associated with ADHD. A recent study found that about 22% of ADHD participants had probable or likely seasonal affective disorder – compared to less than 9% of the control group (without ADHD).
A 2011 study in the journal Nature looked at the circadian rhythm aspects of ADHD through monitoring activity (actinograph), sampling cortisol levels, and by sampling clock genes at various times throughout the 24-hour period in people with ADHD and a control group. The activity monitors showed "significant diurnal and nocturnal hyperactivity in the ADHD group". The clock genes were in a normal rhythm for the control group by that rhythm was lost in the ADHD group. And the cortisol rhythm was "significantly phase delayed in the ADHD group." The study concludes with "These findings indicate that adult ADHD is accompanied by significant changes in the circadian system, which in turn may lead to decreased sleep duration and quality in the condition. Further, modulation of circadian rhythms may represent a novel therapeutic avenue in the management of ADHD." 
To further understand the role of the circadian clock in ADHD, researchers knocked out one of the core circadian genes, PER1, in mice. This disrupted the circadian system and gave the mice ADHD-like symptoms. The study concludes "The circadian model for attention deficiency and hyperactive behavior sheds light on ADHD pathogenesis and opens avenues for exploring novel targets for diagnosis and therapy for this common psychiatric disorder."
Circadian disruption from blue light at night impacts everyone, but for people with ADHD, optimizing circadian rhythm may have multiple benefits: sleep, attentiveness, and overall health.
A study on correcting circadian rhythm in ADHD patients used bright light therapy (10,000 lux) for 30 minutes, first thing in the morning for two weeks. The study combined that with reducing overhead lighting from 4 pm until bed along with wearing blue-blocking glasses. After two weeks of bright light in the morning and blue blocking in the evening, study participants showed a shift in their mid-point of sleep by almost an hour. This was "significantly correlated with decreased ADHD-RS total scores...and Hyperactivity-Impulsive sub-scores". 
A registered clinical trial of adults with ADHD wearing blue-blocking glasses before bedtime showed really significant improvement in sleep quality. But the study didn't actually assess whether the glasses decreased ADHD symptoms... The results of the study did show that the participants with ADHD had a hard time complying with the study parameters, with only 63% of participants completing the 2-week study.
Taking all the research into account, it seems like an obvious solution:
- bright light in the morning AND
- eliminate blue light with blue-blocking glasses in the evening
Bright light therapy devices are available on Amazon. Alternatively – and free! - you could try going outside either immediately after getting up or as soon as it is light out.
Blue-blocking glasses are also available on Amazon with lots of styles to choose from. You are looking for glasses with orange or red tinted lenses that block 100% of blue light (not the lightly tinted computer glasses).
But... the key here may be the fact that the second clinical trial had such a hard time with participants complying with wearing the blue-blocking glasses consistently. The solution may be to automate the process of turning off the overhead lights in the evening and remembering to put on your glasses. There are all kinds of home automation systems now for adjusting your lighting based on the time of day. These are often quite expensive, though. Setting a reminder on your phone (perhaps two reminders in case you ignore the first one!) may be an inexpensive way to get in the habit of putting on blue blocking glasses at night.