1) 10-20 minutes (ideal for boost in alertness and energy);
2) 30 minutes (may actually cause sleep inertia, a hangover/groggy feeling for up to 30 minutes afterward, before feeling restored);
3) 60 minutes (best for improvement in remembering facts, faces and names;
4) 90 minutes (leads to improved emotional and procedural memory and creativity, and typically avoids sleep inertia).
(Published Feb. 19, 2014)