| WAFA | Wake time after final awakening |
| Wake time | Total time that is scored awake in a polysomnogram occurring between sleep onset and final wake-up. |
| WASO | Wake time after sleep onset |
| WASO | Wake After Sleep Onset. The total amount of time you spend awake in bed after sleep onset. |
| Whisper Swivel | One manufacturer's arrangement to vent off the air from the mask end of the system. |
| White Noise | Mixture of sound waves extending over a wide frequency range that may be used to mask unwanted noise that may interfere with sleep. |
| Wilkinson Addition Test | Performance test; numbers added for one hour. Often included in a battery of tests to measure the impact of acute or chronic sleep loss. |
| Withdrawal | Effects experienced when a patient stops taking sleeping pills. |
