The beegees. The song is called lonely nights.
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The Bee Gees
From my little bit of research, it would seem that "Good Morning, Mr Sunshine" was a well-known children's lullaby as early as the '60s, and it has possibly been around since the late 19th century! Thus, it would be difficult to track down the original author.
Maybe you mean Mr. Universe. It was sang by The Flirtations.
If you mean "Mr Blue Sky", it was the Electric Light Orchestra
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