There isn't a direct gene for ADHD or depression, but there is a 'sensitivity gene' that makes individuals more susceptible to stress. According to epigenetics, present and nurturing parenting in the first year of life can effectively neutralize the expression of this gene, preventing future mental illness.

While winter Seasonal Affective Disorder (SAD) is well-known, a reverse pattern exists where summer's excessive light and heat can cause agitation, anxiety, and low mood, challenging the universal view of summer as a positive season.

The impact of estrogen on aggression is context-dependent. During long days with ample sunlight, estrogen does not evoke aggression. However, during short days, which are associated with higher stress hormones and lower dopamine, increased estrogen levels heighten the predisposition for aggressive behavior.

Vitamin D acts like a hibernation hormone, signaling seasonal adaptation. When levels are chronically low, as with modern indoor lifestyles, the brain loses its ability to regulate sleep cycles effectively, essentially 'forgetting' how to sleep. This is why low D levels are linked to profound sleep disruption.

Decades of twin studies reveal that, on average, all psychological traits are about 50% heritable. Crucially, when it comes to pathological personality traits found in disorders, the heritability rate actually exceeds this 50% baseline, indicating a stronger genetic influence for these extreme conditions.

The rise of diseases like ADHD, fatty liver, and depression since the 1980s is not just due to toxins but coincides with a major lifestyle shift indoors, driven by computers, air conditioning, and sunscreen. This deprived humans of essential biological inputs from the sun, leading to systemic health issues.

Early pubertal timing in girls—more so than the rate of development—is a strong predictor of future health risks, including mental health issues, earlier menopause, and a shorter lifespan. This suggests a deep biological trade-off between reproductive maturity and longevity, observed across species.

Large-scale genetic studies suggest many distinct brain diseases (mania, depression, ADHD, Alzheimer's) are not separate conditions. Instead, they may be different expressions of a single, general genetic susceptibility to brain dysfunction, which researchers call "Factor P".

Melatonin isn't just for sleep; its release duration changes with seasonal light exposure. This acts as a hormonal calendar, signaling the time of year to your entire body and influencing major biological functions.