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This phrase is a polite command that translates to "sleep well." Use it primarily with children, family, and close friends. Conjugate the verb dormir depending on the person you're addressing.  Tú: "Que duermas bien." Usted: "Que duerma bien." Vosotros: "Que durmáis bien." Ustedes: "Que duerman bien." Particularly if you're telling someone to "sleep well" and intend it more as an instruction (for example to a child), this phrase is appropriate.  Tú: "¡Duerme bien!" Usted: "¡Duerma bien!" Ustedes: "¡Duerman bien!" This phrase is used to mean "sweet dreams," although the literal translation is something like "that you might have sweet dreams."  This phrase typically is only used for children – occasionally for younger siblings or significant others. Since the phrase is only used familiarly, you only need the familiar conjugations of the verb tener. Use tengas if you're addressing one person, and tengáis if you're addressing several people. You also can shorten the phrase and simply say "dulces sueños," or "sweet dreams." This phrase, typically only used with children, means "dream with the little angels."  This phrase uses the verb soñar ("to dream"), which is conjugated irregularly. However, since it's only used for children, the familiar conjugations are the only ones you need to know: sueñes (singular) and soñéis" (plural).  You can also say this in the command form: "Sueña con los angelitos."

Summary:
Say "que duermas bien" (kay doo-EHR-mahs bee-EHN). Use the command "duerme bien" (doo-EHR-may bee-EHN). Wish someone "Que tengas dulces sueños" (kay tehn-GAHS dool-SAYS soo-EHN-yohs). Try "que sueñes con los angelitos" (kay soo-EHN-yehs cohn lohs ahn-jay-LEE-tohs).