Possible Clues:

• Acquiesce and go to sleep
• Where Cain went to: even Homer sometimes does
• Fall asleep
• Wynken and Blynken's companion
• Auction motion
• Wordless OK
• Why isn't one done?
• St. Nicholas gives one in 'A Visit From St. Nicholas'
• Visibly agree
• Indicate agreement (or sleepiness!)