If Mathematica can use more memory and it is available, it will take it.
About it dying while you sleep, there could be lots of reasons. Try putting in Print and Monitor functions to track what is going on. You can also set the option NotebookAutoSave to True in the Option Inspector so the notebook will be saved after each output. (Format menu - Options Inspector, values for Selected Notebook, Notebook Options - File Options - NotebookAutoSave, True. See the attached notebook for a screen shot.)
Attachment: NotebookAutoSaveScreenshot.nb, URL: ,