Possible Clues:

• The sound of a clock in the mattress?
• Material used to cover mattresses
• Mattress textile
• Material; sounding like a clock
• Strong fabric for mattresses
• Like a timely Timex?
• Material that might come from bomb
• What may be under sheet suggesting it may explode?
• Jerk on the board one hears is always on time perhaps
• Pillow fabric