How can we describe the way the sleepwalking scene in Macbeth makes us feel?
In Act V, scene 3 we learn from the doctor that Lady Macbeth's inability to sleep is mental not physical and that she must cure herself since it was beyond his skill.
Lady Macbeth is wracked with guilt and relives the crimes she and her husband have committed. After they have killed the king and both have his blood on their hands, she innocently says that a little water will wash away the blood, meaning it will wash away the evidence and thus in her mind, the guilt. How wrong she was. In Act I, scene 6, she made a pact with evil and she is now suffering the consequences of that action. She is already in hell.
Macbeth, by his killing of Duncan, has murdered sleep and sleep is necessary because it is healing. But nothing can heal their guilt. For them both, sleep is dead.
For one to truly understand and feel the depth of the scene, it is necessary to see it performed by a skilled actress. I would recommend the Royal Shakespeare Company production (Statford season 1976) directed by Trevor Nunn with Ian McKellen and Judi Dench. A video of this production is available. When you hear the howl of pain emitted by Judi Dench, you know the woman is in deep agony and pain and understand the power of this scene.
There is a slight problem with the wording of your question. You have suggested that "we" respond to a scene in a play together, with one unified reaction. Often, this can be true, but it is just as possible for audience members to react very differently as individual observers. So, if your question is "How can you describe the way that Act V, Scene i makes you feel," here are a few tips.
The best way to respond to the scene emotionally is to see it as it was meant to be experienced, in a live production. It should be pretty easy to watch the scene and make note of your emotional response. If you can't see it live, see a film version. There are some good ones, and you might even be able to watch just this scene, if you search for it on YouTube.
In deciding how to describe your response, make sure to refer to specific moments, specifc actions performed by the actors and/or specific lines of text. This will let whomever you are telling about your response get a clear idea of what exactly it was that you are resonding to.
I have offered some links to suggested film versions to view and commentary from others, so you can see some examples. Good luck!