conundrum /kə-nunˈdrəm/

1. A riddle, esp one whose answer is a play on words
2. Any puzzling question

• sleeping sickness
• sleeping-suit
• sleeping-suits
• sleep learning
• sleepless
• sleeplessly
• sleeplessness
• sleeplessnesses
• sleep like a log
• sleep like a top
• sleep mode
• sleep modes
• sleep off
• sleep on
• sleep out
• sleepout
• sleepouts
• sleep over
• sleepover
• sleep rough
• sleepry
• sleeps
• sleepsuit
• sleep together
• sleepwalk