Sorting is the process where the mind takes time needed to thoroughly digest and categorize the information gained from travel to the astral plane. The sorting process allows information to be recalled when prompted by specific experiences.

How often does this occur?  

One does not regulate or control, nor can the sorting periods be predicted.  What usually happens is that a tremendous amount of information has been digested mentally but the mind has not quite had the opportunity to allow the subconscious to fully experience and understand.  Through dreams during sleep, the conscious and subconscious exchange information, insights, awareness, and understandings.  This is called sorting or sifting.  Eventually the mind has had time to thoroughly digest and categorize the information allowing it to be recalled when prompted by specific experiences. This is usually called remembering.  But the fact of which trigger this sorting or sifting usually relate to how involved mentally the person may feel.  If they are totally consumed with some project, then the information will not be sorted until a later time.  This is detrimental in the fact that there will be some loss of the raw information before sorting takes place. The mind sensing this will try to indicate to you through this desire for sleep before the information is allowed to evaporate. So when you're tired, sleep.