its definitely up to ur personal preference. but I think it is way too dark for a head-only portrait. if trying to depict sadness, definitely need a distinct expression or a distinct action. If the main idea of increasing the contrast to bring out the wrinkles, then it might be slightly overkill. a bit too much i think... this pic shows more of the toil of the ahpek rather than sadness.
here's a picture of my granny.. picture (for reference and not for comparison)
bottomline is... if u think this is what u want to portray then stick to it. don't be swayed by comments of others.