Stress is a big issue in reptiles, there are a million reasons for animals to become stressed, and a million symptoms. Although you have to remember, many reptiles are prey animals in the wild and do their darndest to hide their weakness. If you are worried about stress...first...do a run down of your husbandry.
How are your temps? (highs 88-90, lows 80-85)
What do you have for hides? (humid, warm dry, cool dry for each lizard)
Are the lizards in a high traffic area or are they harrassed by any other pets? (dogs, cats etc)
What is their substrate? (anything but sand or bark or walnut shells)
Do they have a good day/night cycle?
Are they fed a proper diet with good supplementation?
If everything checks out, think about their behavior
Are the leos males or females?
How old are they?
How long have they been together?
Is their recent behavor different than you are used to seeing? if so, when did the change occur?
Signs that a leo is stressed could be:
Loss of appetite or complete anorexia, hiding all the time, taking the submissive posture between two animals (the one on the bottom if they are lying on top of each other...the second one to eat at feeding time,etc), runny or smelly stool, weight loss or signs of outward disease. Sometimes though, they are good at hiding the signs.
If your leos are eating well, not losing weight, i wouldnt worry too much. It is definately normal for them to be hiding during the day and out at night. Remeber, its always a good idea to get fecals done at the vet, and this would assure you that there are no parasites causing or caused by any stress. 

-----
0.1 Bearded dragon (Hannabil)
0.1 mexican Black kingsnake (Morticia)
1.1.2 Leopard Gecko's (Pogo, Louise, Orion, Jeffrey Nothing)
0.0.1 Tokay Gecko
1.0 Blue Tongue Skink
0.0.1 Reverse Okeetee Corn (Autumn)
0.1 Bullmastiff (Asha)
4.1 Cats (Poe, Tucker, Abhid, Felicity, Emmy)