What is welsh for sleep tight?

There isn't a direct translation of "sleep tight" into Welsh. It's a phrase that doesn't have a literal equivalent in Welsh.

However, you could use these phrases to express a similar sentiment:

* "Cysgu'n dda" - This translates to "Sleep well" and is a common and polite way to say goodnight in Welsh.

* "Hwyl fawr a chwsg da" - This means "Goodbye and sleep well," which is more formal and adds a touch of warmth to the sentiment.

* "Cysgu'n dawel" - This means "Sleep peacefully" and is a more poetic way to wish someone a good night's sleep.

The best phrase to use would depend on the context and your relationship with the person you're speaking to.