21551
Using female Siberian hamsters ( Phodopus sungorus ) , we investigated the effects of chronic nightly exposure to dim light on daily rhythms in locomotor activity , serum cortisol concentrations , and brain expression of circadian clock proteins ( i . e . , PER1 , PER2 , BMAL1 ) .
 cortisol
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: Chemical 
Text: {0} 
Answer:
 cortisol