> Three minutes of exposure to deep red light once a week [from a flashlight-sized light], when delivered in the morning, can significantly improve declining eyesight 670 nm red lights are commonly available. [Weeklong improved colour contrasts sensitivity after single 670 nm exposures associated with enhanced mitochondrial function](https://www.nature.com/articles/s41598-021-02311-1). They specifically address whether normal sunlight would have the same function: > The energies at 670 nm that we have used are within a log unit of those that can be found in sunlight. This naturally raises the question of whether daylight may impact positively on mitochondrial function in the retina. While this may be the case, we are only recently coming to appreciate the optical absorbance characteristics of mitochondria and their functional consequences. **Mitochondria [also] strongly absorb at shorter wavelengths that are present in environmental light and these are known to reduce their function.** Consequently, long, and short wavelengths in sunlight may have opposing and perhaps balancing influences on mitochondrial function in the retina. It goes into details of the lights used.