B737-800 flight crew reported that a passenger door was not fully closed during takeoff roll.

Narrative

We were cleared for takeoff on runway XX at ZZZ. After taking the runway; I relinquished aircraft control to the FO; who performed the takeoff as Pilot Flying. The FO applied takeoff power; and shortly sometime after; they announced Master Caution. I could not see the Master Caution light lit on my side; and quickly did a cursory check of my Recall lights; saw nothing lit; and looked across to the FO's to barely see his Master Caution lit; with no associated Recall lights lit either. After doing a quick search for any light lit between the overhead panel down to the center console; and still unable to see any lights lit up; I performed the rejected takeoff procedure. While taxiing off the runway with the airplane under control; we looked up to see the FWD Entry Door light lit up on the overhead. After clearing the runway; I phoned to the Flight Attendants to inquire on the status of the L1 door handle; to which they acknowledged that it was indeed not fully stowed. Of note; my normal brief every flight with new flight attendants includes to push down on the handle after the gate agent shuts the door. After discussing with the FO; we noted that the angle of the sun (left rear in the afternoon; taking off to the north) into the cockpit greatly affected our ability to see any lights that were lit up. I recognized that I couldn't even see my master caution lit up; and I was surprised the FO saw theirs lit up.Cause: FA not ensuring the handle was pushed all the way down after door closure; and the angle of the sun into the cockpit making it hard to see the nature of the master caution we were confronted with. We did not see the door light until turning left off of runway XX in ZZZ; making the sun angle during that time of day at this time of year a concern to be noted.