2608
Participants underwent polysomnographic sleep recordings on days 1 to 3 , 7 to 9 , and 14 to 16 ( first , second , and third weeks of abstinence ) .
 None
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: Disease, Chemical 
Text: {0} 
Answer:
 None