In Tehillim 121, David says of Hashem "הִנֵּה לֹא-יָנוּם, וְלֹא יִישָׁן שׁוֹמֵר, יִשְׂרָאֵל." At first glance, this appears redundant, נום means slumber and שנה means sleep. What is the difference between נום and שנה as they are used in Tehillim?
Metzudat Zion on Psalms 121:3:2:
ינום . מלשון תנומה והוא שינה מועטת וקלה :
The word יָנוּם comes from the (noun) תנומה meaning a light and "short" sleep.
In other words, שנה is a longer sleep, like what you do at night. תנומה would be what some call a "twilight" sleep. Like the "sleep" you do at your work desk, sometimes. (Of course, it's not specifically YOU :-)
The point is, that David is emphasizing that G-d always watches Israel and doesn't even take this "lighter" sleep. He's always awake watching us. (Only G-d can do that!)