Photon noise isn't affected by ISO since it is "real" but can improve by using longer exposures. The more collected, the better the signal/noise ratio. But upping ISO can improve the shadow parts of a shot while having little negative effect on the brighter parts.
I only need to figure out when what works best. For my night shots, I can halve the shutter or double the ISO for the same exposure but it might lead to different quality.
Generally the longer and wider at the lowest ISO will generate the best signal/noise ratio and thus better quality but I'm not sure when it hits its maximum benefit. From a practical point, upping ISO is better at night. Shutter quickly becomes time-consuming; 30s, 1min, 2min... and that doubled if LE-NR is used. Upping ISO is much faster.
But since upping ISO increases read noise, I first have to find out what cost/benefit there is.