We mostly say; "welterusten en slaap lekker!"
Yes, "welterusten" (good night) is an interjection used to wish someone a good night's sleep, literally a wish to "rest well." It's a combination of the words "wel," "te," and "rusten" (rest).
"Slaap lekker" is Sleep well, so technically we say it twice... Hmm lol
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