Much of the light emitted by the sun is in the red and near-infrared spectrum. As a result, sunlight exposure might be able to replicate some of the effects of red light therapy.
It’s estimated that unobscured sunlight delivers around 20–40 mW/cm2 of red and near-infrared light (although the amount varies depending on factors like time of day, time of year, and latitude).[1] This puts sunlight in the range of the power densities used in many red light therapy studies.
That said, there isn’t much direct research looking at whether sunlight actually does replicate the health effects of red light therapy, so whether sun exposure can act as a form of red light therapy remains uncertain.
Furthermore, sunlight is a major source of other wavelengths of light as well, which could enhance or inhibit the effects of red light. For example, while exposure to red and infrared light may reduce signs of skin aging (although the evidence for this is weak), the net effect of getting a lot of sunlight exposure is the promotion of skin aging due to the effect of ultraviolet light.[2][3] With that said, it may be worth noting that most sunscreens block UV light while not blocking much red and infrared light (although a small number of sunscreens block both).[4][5][6]
Ultimately, whether the net effect of sunlight is similar to that of red and near-infrared light can’t be assumed.