This great article deserves a bump. I might mention that in addition to the safety benefit of carrying overnight gear for a planned day hike is the peace of mind benefit for your family.
If you tell them that you have planned for an overnight trip but will do it in one day if all goes well, they won't worry if you take two days. On the other hand if you tell them it's a day hike then at some point they start to worry and call S&R and they mobilize at midnight. Not a good situation.
When I was a teenager my folks and I developed a system that was basically a flight plan. They knew what gear I had, how much food, planned route and planned completion date. I always carried extra food and we had a standing agreement that they would alert S&R at noon when I was a day late. That seemed like a reasonable safety cushion and gave enough time that S&R could mobilize and make it to the trailhead in time to interview hikers coming out that day. We thought this was a better system than calling S&R out in the middle of the night when they couldn't really do anything other than starting the mission sleep deprived.