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 synergizes 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 in-game hour will restore 12 health.
Due to the Strength increase when the perk is in effect, if one is within 20 pounds of being overencumbered, they will become overencumbered upon entering any indoor area or after 6PM unless their 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 HP per second.
“Catch some rays! A good perk for the extra melee damage, carry weight, Health, and Action Points you receive. When you're finally able to regenerate Health, combine this Adamantium Skeleton (which heals limb damage) and snoozing on a bed during the night. Focus all your exploration during daytime hours.”— Fallout 4 Vault Dweller's Survival Guide description
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. This effectively renders Lead Belly useless, as any small amount of rads from food or drink are nearly instantaneously absorbed. Rank 3 heals the Sole Survivor by 0.5% of their 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 receive any further benefit to stats.