Dweezil Zappa

We are remodeling the kitchen and dining room and the designer is suggesting a couple of skylights. We like the idea for light, but I am concerned about long term leaks, as well as overheating in the summer and heat loss in the winter. The roof they will be installed on is mostly south facing and there are no overhanging trees. Winters can be quite cold and summers quite hot. We do not have air conditioning.
A Google search to see if modern high efficiency windows will reduce heat gain/loss seems to yield no clear answer. What do you think?