There are several synonyms for the expression "not sleeping a wink" that can be used to convey a similar meaning. One common phrase is "tossing and turning," which refers to the inability to get comfortable enough to fall asleep. Another phrase is "lying awake," which suggests that the person is physically still and in bed but mentally alert and unable to sleep. "Insomniac" is a noun used to describe a person who suffers from chronic sleeplessness and may be unable to sleep a wink. Lastly, "restless" is an adjective that can be used to describe someone who is unable to settle down and relax enough to fall asleep.