Originally, when I heard a lecture on this topic, the professor said that since light is the strongest zeitgeber he suggested driving into work without sunglasses on. This would allow for stronger signals to be sent along the retinohypothalamic tract to stimulate the SCN and increase your morning cortisol production at a faster rate. I still tend to drive with them on since I feel blinded by the sun in the morning. However, on mornings when it is partially cloudy out and I did not get a lot of sleep, I drive with them off because this will help me feel more alert than if I was shielding what little sunlight was available.
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