medical benefits of sunlight

[bosun]

The following sentences discusses the medical benefits of sunlight and can you please explain the sentence, especially, the red parts in other words? Especaill i am not sure what ' which ' referts to.

Dr. Leonard Dodds says that after a long winter, the return to the sun is a general stimulus to the body, more potent if applied after a period when it has been lacking which gradually loses its effect if exposure is over prolonged, even when not excessive.

[konungursvia]

After getting no sun, going back into sunny weather and getting sun on your skin is good for the body, especially after getting too little sun. Getting sun exposure loses its power if you get slowly tanned. But don't overdo it.