I can’t tell you where this expression comes from, but it refers to someone being happy, refreshed, and ready to take on the world after a good night’s sleep.
e.g. The baby finally slept through the night, and I woke up bright-eyed and bushy-tailed.
This expression, however, is often used ironically, particularly to refer to someone who looks rough after a poor night’s sleep or pulling an all-nighter and staying awake all night.
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