Unlike Night Person, which operates both indoors and out, this perk only operates outdoors. It is in effect between 6 AM and 6 PM. It synergises well with Rad Absorption for healing minor health damage and minor radiation poisoning while exploring or fast traveling.
In Fallout 3 with this perk active, waiting for one hour will restore 13 health.
Due to the Strength increase when the perk is in effect, if you are within 20 pounds of being over-encumbered, you will become over-encumbered upon entering any indoor area or after 6PM unless your Strength (without the bonus) is already 10. This includes bonuses from equipment, chems, other perks, etc.
It used to regenerate only 1 hp every 10 seconds. The speed was greatly improved in a later New Vegas patch to 1 health per second.
Rank 2 heals roughly 4 rads per second, meaning the Sole Survivor can go from the brink of death to completely radiation-free in approximately four minutes. Rank 3 heals the Sole Survivor by 0.5% of your total health every second.
In Fallout 4 SPECIAL stats can be meaningfully boosted past 10, so the bonus stats will be in full effect, whereas in previous games one could easily have taken this perk and had a character with stats and equipment high enough to not recieve any further benefit to stats.