Types of Short Sleep Expression

Short sleep seems to happen in different ways for different short sleepers.  My family appears to have both forms.

• The well-known form is the (6Hr) form where the sleep time is regular "11:08pm" and the wake time is natural, and regular "5:00am" without alarm approximately six hours later.  Once awake the short sleeper is alert and productive for the rest of the day.  
• A less studied form is the (INS) insomniac form where the short sleeper sleeps 1-3 hours at a time and wakes naturally and is alert and productive for several hours and returns to sleep for another 1-3 hours waking naturally without alarm. The rest of the day the person is alert and productive.  In college I went to sleep at 10:00pm woke naturally at midnight, did my homework till 4:00 and slept until 6:00 waking naturally without an alarm so reliably that I had the house wake up chore. 
• I have heard of an irregular 1-3 hour form where alert intervals are irregular and may exceed 24hrs.  The sleep time when needed is regular.

A niece has a large three generation family where short sleep is the norm.  Grandfather was 6Hr.  Grandmother was INS from her grandmother.  "Another middle of the night letter, Dorothy."  Due to the strong reinforcement of hDEC2 most descendants carry the modification.