Why Sunbathing Makes You Tired: A Circadian Perspective

You’ve felt it before.

After spending hours sunbathing, especially during midday, you start to feel drowsy, sluggish, ready for a nap. I have experienced this countless times, and it’s led me to ask questions which sent m down the rabbit hole.

Most people chalk it up to “heat exhaustion” or “dehydration,” but there’s a deeper, more intelligent system at play. What if the fatigue you feel from sunbathing isn’t a sign of damage, but a natural signal of circadian alignment?

Let’s unpack why UV light and full spectrum sunlight make us tired, through the lens of circadian biology.

When you're outside during the day, your suprachiasmatic nucleus (SCN) receives potent input from the sun via the retina. This tells your body that it's daytime and to be alert.

What’s less appreciated is how this cascade is not linear, it’s cyclical.

The rise leads to a n…