Hi! Aside from making sure something in the viv hadn't changed (like the
UVB bulb hasn't exceeded its 6-month life, the basking temps are staying in the appropriate ranges of 100-115F,
<<<< BAD ADVICE !!!
>>>> 115F (46 degC) WAY TOO HOT !!!
<<<< never allow your bearded dragon to be exposed to temperatures higher than 43 degC (109F)
<<<< see this for reasons why : viewtopic.php?f=5&t=233516
>>>> 46 degC and above is LETHAL temperature range for bearded dragons
>>>> temperature above 44 degC is critical and dangerous to beardied dragons
>>>> 41 - 44 degC is a temperature range outside a bearded dragon's comfort zone they will not voluntarily stay there and will be frantically seeking shade or a cooler place
If he's dark , he is most likely too cool and trying to warm up. Check your zone temperatures and basking spot temperature using an ACCURATE digital thermometer.
the cool side is under 85F, the substrate isn't too dirty [which is almost impossible to clean if the substrate is sand]), I wouldn't worry too much about the beardie's coloration unless they're showing other signs of illness: not pooping/eating regularly,
black bearding, frequent coughing, etc. Bearded dragon "stress lines" aren't actually the best indicator of stress, sometimes they just show up. They usually darken themselves in order to soak up more light/heat, so maybe it would help to raise the temps a little bit or switch out the
UVB bulb if it's nearing the end of its 6-month life. (Also, if your UVB isn't a fluorescent tube Reptisun 10.0 or an Arcadia 12%, keep in mind that other UVB bulbs have a reputation of not emitting a reliable amount of UVB.)