I wonder if you would care to comment on my attempted translation. It's a request from my younger brother that I (presumptuously) accepted. Here's what I came up with:
Somni dummodo ne dormeas. -or-
Noli dormire nisi somniorum causa
The intended meaning is: "Dream, but don't Sleep"
Thank you very much.
“Somnia, at noli dormire" is the correct translation for “Dream, but don't Sleep”, if this imperatives are addressed to only one person such as “Rêve, mais ne dort pas” in French.
Otherwise, you must say: “Somniate, at nolite dormire”, if the imperatives “Dream, but don't Sleep” are addressed to many persons such as “Rêvez, mais ne dormez pas” in French.
[Read more below].
As for your translations “Somni dummodo ne dormeas” and “Noli dormire nisi somniorum causa“, I am sorry, but they both are wrong.
-Dream = SOMNIA (2nd.person singular, imperative of SOMNIO, I dream) or SOMNIATE (2nd.person plural, imperative of SOMNIO)
-but = AT (conjunction introducing a direct opposition)
-don't =NOLI (2nd.person singular, imperative of NOLO, I do not wish/want) or NOLITE (2nd.person plural, imperative of NOLO).
Note that the imperatives NOLI and NOLITE with the infinitive are used to express the English imperative “Do not..”
-Sleep =DORMIRE (infinitive of DORMIO, I sleep)