I have my rear tail light on day or night. On cludy gloomy days having it on makes me a lot more visible. I'll turn the front one during the day if the weather gets a bit gloomy.
I agree Nigel. I leave my Niterider Omega 300 (numbers mean lumens of course) on flash mode during the day. With people driving distracted, the flashing the light will get their attention when they look up from texting for a half a second...at least that's the idea! But it's better than no idea!!
I've read two different studies on flashing lights vs non-flashing lights, Europe said that flashing lights made it difficult for motorists to ascertain how far they were from a cyclist, so they outlawed flashing lights of any sort. Canadian studies showed that flashing lights attracted the attention of motorists better than a steady light, so they allow for both, flashing and steady. I took those two studies and combined them! During the day I run with a 400 lumen Lezyne Hecto Drive 400XL on strobe mode, and the NiteRider Omega 300 on highest flash mode; but during the night I ADD 2 more lights. So at night the Hecto Drive is left strobing but is put on my helmet instead of the bars, I then add a Phillips Saferide 80 to the bars and that one is on steady (it doesn't have a flash mode anyways, but it's far brighter then the Hecto Drive is when it's on steady). Then I leave the Omega 300 on my seat bag and put it on the brightest steady mode, and I add a Niterider Aero 260 to my helmet and put it on alternating flash mode. If I end up on a dark country road or bike path I will usually turn the Hecto drive to steady mode as the strobing is annoying to me in the pitch dark, plus I can aim the helmet light in any direction I want to see something better, also the higher angle of the helmet light helps to highlight road imperfections so you can see a pothole with more detail, handlebar lights tend to make potholes look less deep leaving a rider to think it's not a big deal, but the higher up the light is the more depth those potholes take on. This is why true 4x4 drivers have those driving lights on light bars located above the cab of the vehicle.
The front light on during the day, not sure if I really need to do that, I do it anyway, but my thinking is that I have more control over what happens in front of me then I do behind me, so I really don't think the daylight front light is really all that necessary, but I do it anyway just in case. So if there ever was an accident and the motorist stops, which most do not, but then the cop can't say I wasn't doing everything I could to be seen.
And that's all the lighting I use. At night I do add ankle reflectors, and I add a Home Depot neon green mesh safety vest with wide reflective bands on it. Kind of nerdy that vest is but my safety is more important than how I look to others, I'm not riding to put on a fashion show!!