Live Photos is the newest feature in iPhone 6s and 6s Plus which records a short video, before and after a photo is taken. This makes for some memorable moments. However, as per the discussions going around the web, it seems that turning on Live Photos effects low light photos as they appear darker.
The reason seems to be that when Live Photo is enabled, the camera records a video, then takes a photo before turning back to recording a video again. Between this, the exposure has to change, resulting in a slighter dimmer photo than without Live Photos turned on. Turning off Live Photos allows for lower shutter speed which is important to capture more light in dark conditions. Also, the exposure values are normally different in the same environment for photo and video capture so perhaps the adjustment is what is causing some users to take notice.
Many users on Reddit have shared this experience and 9to5Mac also reports that this is happening. For now, if you believe you are seeing more noise and lesser detail during low light photography, try turning off Live Photos to see if it brings any improvement. The view finder might not show a difference so compare in the Photos app once you capture them.