Why does laser hair removal sessions have to be done 4-8 weeks apart?

Great question.  You are correct that laser hair removal needs to be performed during the active growth phase. Hair goes through 3 cycles, one of which is known as the growth phase.  When you receive a treatment, most of the hair in the growth phase will be effectively treated by the laser and those hair follicles will stop producing hair.  However, not all hair is on the active growth phase at any given time.  Depending on the body part, the hair phases change about every 4-8 weeks. If you treat too soon after a laser session, new hairs will not have entered the active growth phase making the treatment less effective.  Essentially, you would just be retreating the same hairs you had already treated, the ones that the body is still working on eliminating. 

Thank you for your question. At any given time, almost 90% of your hair is in this phase, but because not all of your hair is in this phase at once, it requires several treatments to achieve maximum results. For more information, please get a in person consultation from a board-certified dermatologist. 

thank you for your question. The time that we suggest between laser hair removal  treatments  is to achieve the destruction of as many hair follicles as possible. If you had your treatment two weeks earlier than suggested, your treatment would not be as efficient as it could be. This would probably cause the need for additional treatments and additional costs to you.  I hope this helps. Regards, Dr. DiPasquale

this is a very good question.  Hair removal needs to be done in a series because hair grows in cycles. The hair will only respond to the laser light when it is in the active growing phase.  Depending on what area of the body that is being treated -the active growing phase varies.  You want to get as many hairs in the active growing phase as possible   in order to get an effective treatment.  The reasoning behind waiting a full growing phase is that more hairs will be amenable to treatment.  

Thank you for your question! 
Yes, the Laser Hair Removal (LHR) treatment only kills follicles in the growth phase, you are correct. About 25% of the follicles are in this phase at any one time. 
The follicular generation process is continuous. The reason for waiting 4 - 6 weeks between is this will give the best opportunity to have a higher number of vulnerable follicles rather than, say treating at 1 or 2 week intervals. 
The follicles that are still active will continue growing hair. 
In our practice people normally start to see improvement after the 2nd treatment, one treatment doesn't do much, you have to stick with it to continue to see improvement with reduction of the active follicles. 
I hope this helps!