DO they recover and lose their sag? Be careful not to overwater.

John

If they hang their head, they could have wet feet.
Sounds like over-watering.

Do you have dry brown tips/edges on the leaves? If so, your problem could be over-fertilizing, or a combination of both over-fertilizing & over-watering.

You can't rule out HEAT. If you are using heat mats (which isn't advisable if you have leaves) and the roots get too warm, that could contribute to the problem.
If you are using HID lighting in close proximity to the seedlings, the ambient temperature could pose a threat.
Look for anything that could promote stress.

Just several things to consider given the amount of info you supplied.

You didn't mention the growing environment but If you had them in a greenhouse, or even window and lately the sun has been more intense, the shock from excess UV light may be causing the leaves to wilt during most intense sunlight. Gradual adjustment to the direct sunlight (Hardening off)may remedy this. But I tend to agree with Rhody that fertilizer (high soil salt) may be involved. Replanting into fresh potting mix or flushing soil with excess water should help.

Also your plants may already be rootbound, depending on the pot or cell size. If so, the roots dry out very quickly after watering. The best remedy for this is to transplant to a larger pot.

Last year I noticed my Rocoto leaves hanging down every night. First I was worried thet there is something wrong with it, but then I noticed that it is normal. I guess it went to "sleep" at night.

Are the leaves in your peppers hanging down all the time or does it happen at certain time? Are the leaves soft and wilted? My Rocoto leaves were not wilted, but the difference in the position was noticeable.

Here is a link that might be useful: