"A long rest is a period o f extended downtime, at least 8 hours long, during which a character sleeps or performs light activity: reading, talking, eating, or standing watch for no more than 2 hours." -- PHB, page 186.
That "2 hours" can be interpreted in a couple ways. Either your Long Rest can only include at most 2 hours of "light activity" - the rest of which is sleep - or you can only stand watch for no more than 2 hours before it becomes "strenuous" and you can spend the remaining 6 hours doing other things.