Related Clues:

• Deep groove
• Monotonous practice
• Sheet music?
• One way to describe the outfit
• Sleeper's noise
• Boredom indicator
• Tiresome grind
• Concavity
• It might wake you up
• Make noise in bed
• Produce Zs
• Long groove
• Sleep interruption
• Endless 9-to-5 job, e.g.
• In comics, it's represented by a string of Z's