It's nice, but it's totally impractical as actual sleepwear. You'd strangle yourself with the pearls, if you're not lying on them and getting the Princess and the Pea effect going. And your arms would be tangled in the huge cape so you couldn't free yourself. Basically, it's a walking deathtrap. In fact, it's almost one of these, as recounted in this tale of woe:

While delving in the Lost Caverns of Tsojcanth, old Fingers spotted a rich leather cloak hanging from a peg. While his allies fought off fire bats, Sticky Fingers plucked the cloak from the hook and threw it over his shoulders. To his shock, the cloak came alive and enveloped him, killing Sticky Fingers before any allies could help.

— quoted in this article, monster #10.

Still want to make your own one of these? Check this awesome Star Wars costume site by Maggie: The Padawan's Guide to Star Wars Costumes. Padmé's nightgown details are here.

Transcript

R2-D2: What's that you're drawing, Sally?
C-3PO: Padmé's nightgown!
GM: Nightgown? Well I suppose we can skip ahead to the evening.
C-3PO: Look, it's made from steel blue silk satin. And it's got diamond hair-loom stitched bias-cut panels.
C-3PO: And here I've drawn some spaghetti bolognese straps. And three shiny strands of pearls that fell off her shoulders, joined to a silver brooch.
C-3PO: With a waterfall cape at the back!
R2-D2: Whoa.
Obi-Wan: If you studied your schoolwork that hard, you wouldn't—
C-3PO: Ben! You better not tell!
Anakin: It's very pretty. I'm sure Padmé looks beautiful in it.
Padmé: Hey, draw an armour-plated night-vision visor to go with it!