How do I determine whether my EEG signal needs denoising?
Your recording necessarily has noise. That's a property of any physical apparatus, and you are recording the activity of noisy neurons. The other property of noise is that, by definition, it will average out. So don't do anything about that. 100Hz is quite a low frequency. Artifacts (e.g. blinks, but I doubt you would have that in sleeping subjects) would typically have a higher frequency than that so (1) it's unlikely it has been recorded by your system at all, (2) you can low-pass filter your signal to remove both the 50Hz artifact and possible other artifacts. Other artifacts such as sleeping position would have very low frequencies. So you can high-pass your signal to get rid of them. I'm not familiar with sleep EEGs but I would not worry too much about it. You can also check few papers and see how they filtered their signal.