Air carrier Captain reported descending below glide path on a visual approach to SFO when situational awareness was lost.

Narrative

On a night FMS bridge visual to Runway 28R to SFO; we got significantly low on the glide path and had a glide-slope warning. I was the Pilot Monitoring (PM) and the FO was the Pilot Flying (PF). I thought we were in LNAV VNAV; on the approach profile with the ILS frequency also plugged in. I'm thinking we maybe we were in level change based on how low we got. The PF did not disengage the autopilot until we realized we were significantly low. How low I'm not sure; but the PAPI/VASI was showing 4 red when I snapped my attention to the situation. The VSD also showed us significantly low. As soon as I looked out front; I said to the PF to 'get two white lights on the PAPI'. He leveled off and added power. I thought of taking the aircraft but he was making excellent corrections. A go-around could have been warranted but we elected to land as we intercepted the glide path. I'm glad I had the VSD up; but I did not use it early enough as the PM to avoid this situation.Causal factors: Fatigue; we were rested but it had been a long 5 hours since [departure]. Complacency; it was perfect weather and both thought LNAV VNAV was a great plan to have some guidance; then followed the flight director too low in altitude. Experience; the PF was highly experienced in the Aircraft X and I was somewhat inattentive or complacent as the PM. I assumed all was good. Distraction; I did not have my head fully in the cockpit prior to our first indication of an issue. This was 100% human factors. I don't think we got ourselves close to an incident but getting low is a never good on an approach. Ultimately; I failed to monitor the flight path and we ended up in an undesirable aircraft state.