There are many ways to measure human temperature and the choice of location and device really depends on your application. For example, if you’re interested in looking at circadian rhythm, the core temperature begins falling shortly before typical bedtime but the wrist skin temperature would actually be rising because the main cooling mechanism is pumping the blood from the core to the extremities. Core temperature would be the most useful but as mentioned above hard to get. In research studies, the distal-proximal skin temperature gradient (ie hand-trunk difference) is used to get a proxy measure of circadian rhythm (brief review paper http://www.chronobiology.ch/wp-content/uploads/publications/2001_16.pdf).