Okay. How many gallons is this tank? As coursair said, tetras need groups as most are schooling fish. 2 fish here and 3 fish there isn't going to help them any. However, that being said, you've had these fish for many months now if you got them in the summer. Have they always hid? Have you ever observed them eating? Tetras are not bottom dwellers, nor are they picky. I have neons, cardinals, and rummynose in my tank and they eat anything--flakes, shrimp pellets, algae wafers, frozen mosquito larvae, daphnia, and bloodworms, as well as any blanched veggies I add for my catfish. They were shy for the first couple days, but they swim the tank without issue during daylight hours now.
If yours were acting normal and aren't anymore, how long has this behavior been going on? They may have some sort of disease or parasite. If they have been this way since the beginning, and yet are still alive, they may just not be used to your presence and are probably eating when you're not around. If you have that much food left in the gravel, you are probably feeding too much and/or too often. A tiny pinch of flakes once a day or even every other day would be best. When feeding pellets, just put one in for that small number of fish. I have 35 fish and feed them 4 shrimp pellets once a week.
As for light, you can get a cheap timer from a big box store such as walmart for only a few bucks. Set it to what you want and then they will have a routine lighting schedule.
Also, when you say the “water is normal” do you have a test kit for things such as ammonia, nitrite, and nitrate? If not, invest in a good test kit such as the API master kit. It's worth it to know. How much water do you change out each week? Do you use any water conditioner? Do you temp match the water? Fluctuating temps can be lethal. As can chlorine found in tap water.
