Should I microwave-defrost sandwiches?
No matter what you do in this situation, you are likely to end up with soggy sandwiches.  This has nothing to do with light.  Firstly, the bread of pre-made sandwiches will naturally pick up moisture from the filling and condiments.  Often, even freshly made sandwiches will become a bit soggy by lunchtime.  You've also created a further disadvantage by freezing.  Freezing and thawing disrupts the cell structure of foods.  When this happens, moisture is often lost. Of course, the bread will absorb this moisture.  A microwave will thaw and warm, but will not remove much of this moisture. You would have better luck with an oven, but that would be more time consuming.  Really, your optimal approach is to prepare the ingredients in advance, then take 2-3 minutes before you leave for work, and make a sandwich.