Which of these forms is correct

[senthil]

in higher concentrations during sleep than during waking hours
in higher concentrations when asleep than when awake


I feel the second form is correct , If I am wrong please explain

[RonPurewal]

this depends on context.

the latter version requires the SUBJECT of the sentence to be the one who is either 'asleep' or 'awake', because those descriptors don't have their own subjects. so, for instance, the following is correct:
people have a higher concentration of melatonin in their bloodstream when asleep than when awake (because people are asleep / awake)
but the following is incorrect:
the concentration of melatonin in the bloodstream is higher when asleep than when awake (because the concentration isn't asleep or awake)

the former version should be used if the subject of the sentence is NOT the entity that is asleep or awake:
melatonin is present in the bloodstream at a higher concentration during sleep than during waking hours

--

finally, note that the plural ('concentrations') is highly suspect here. unless you're actually talking about more than one concentration (i.e., the concentration of some thing X and the concentration of some other thing Y), you should use 'concentration' in the singular, as is done above.