---
title: 'How Sunlight Protects Against Flu'
source: 'https://youtube.com/watch?v=p6zNt7HUCrY'
video_id: 'p6zNt7HUCrY'
date: 2026-09-27
duration_sec: 89
channel: 'Andrew Huberman'
---

# How Sunlight Protects Against Flu

> Source: [How Sunlight Protects Against Flu](https://youtube.com/watch?v=p6zNt7HUCrY)

## Summary

This video explores why influenza peaks in winter, challenging common explanations like cold weather and indoor gatherings. It highlights the 2009 H1N1 pandemic as a natural experiment that uncoupled the flu from winter, allowing Harvard researchers to identify sunlight as a strong protective factor.

### Key Points

- **Winter Flu Season Questioned** [00:00] — Influenza season occurs in winter, but the speaker questions whether temperature, indoor gatherings, or daylight are the real cause.
- **2009 H1N1 Pandemic as a Natural Experiment** [00:30] — The 2009 H1N1 pandemic peaked in summer, across areas with varying humidity, temperature, and sunlight, providing unique data.
- **Harvard's Sunlight Finding** [01:18] — Harvard analyzed solar radiation data from the pandemic and concluded that sunlight strongly protects against influenza.

## Transcript

The problem is we always have this influenza season, which is in the wintertime. We can talk about why that might be. I would suggest to you that it's because that's when we have the shortest day of the year. But we also have other things that happen at that time. What else is happening?
We're having parties at that time. There's Thanksgiving. There's New Year's. It's cold. You know, does the temperature have anything to do with it? We're also inside because it's cold. So what is it that's actually doing it? Well, 2009 was a banner year because 2009 was the year that we had the H1N1 pandemic.
And that was a boon for us. The reason is because, not because of the deaths that we had, but from a scientific standpoint, this epidemic actually peaked in the summertime. And it was in areas where the humidity was sometimes high, and sometimes it was low, and sometimes the temperature was high, sometimes the temperature was low, sometimes the sun was out, sometimes it was cloudy.
In other words, we uncoupled the influenza virus in 2009 from it being in the wintertime and all of those things that were associated. So now we have all of these data points.
And so Harvard went to work at looking at all these data points. They looked at solar radiation data at that time. And they were actually able to look specifically where that person came from and what was the solar radiation in that particular area.
And what they came up with was absolutely. They said, quote, sunlight strongly protects against getting influenza.