When a guy says "sweet dreams" to a girl, it is typically a kind and caring way of expressing his good wishes for her before she goes to sleep. It's a simple, affectionate gesture meant to convey that he wants her to have pleasant and peaceful dreams while she rests.
In some cases, saying "sweet dreams" can also indicate that the guy cares about the girl and wants her to feel happy and safe, even in her subconscious state. It may be a subtle way of showing that he is thinking of her and wants her to have a good night.
However, it's important to note that the meaning and intent behind this phrase can vary depending on the nature of their relationship and the context in which it is said. If they are close friends or romantically involved, it might carry a more significant emotional weight. On the other hand, if they are just casual acquaintances or colleagues, it may be a friendly and polite gesture without any deeper implications.
As with any communication, the specific meaning can be better understood by considering the overall dynamics of their relationship and the context in which the words are spoken. If you're unsure about the intentions behind the phrase, observing how the guy behaves in other situations and having open communication can provide more clarity.