Do you think the sisters ever sang each other lullabies? Also, how come Luna gets less sleep than her older sister?

Probably, and I'm not sure.

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Do you think the sisters ever sang each other lullabies?

Perhaps. It's kind of a sweet thought.

how come Luna gets less sleep than her older sister?

Maybe she just doesn't need as much as most ponies do. I wonder if she trained herself to live like that when she took on her Princesshood.

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Okay, that makes sense.