The PHB's description of long rests states:
A long rest is a period of 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. If the rest is interrupted by a period of strenuous activity—at least 1 hour of walking, fighting, casting spells, or similar adventuring activity—the characters must begin the rest again to gain any benefit from it.
I can see this as being read one of two ways:
A character can spend 8 hours sleeping, or they can spend 8 hours reading, talking, eating, or standing watch. If they stand watch, it can be for no more than 2 hours.
A character can spend 8 hours long resting, most of which is spent sleeping and up to 2 hours of which is spent performing light activity.
The Xanathar's Guide to Everything optional rules for the woodcarver's tool proficiency (on p. 85) provides a little more information:
Craft Arrows. As part of a short rest, you can craft up to five arrows. As part of a long rest, you can craft up to twenty. You must have enough wood on hand to produce them.
To me, this suggests interpretation #2, because if a short rest of 1 hour yields 5 arrows, surely a character could produce more than 20 arrows over the course of 8 hours.
If true, this would suggest that adventurers can truck along on 6 hours of sleep per night, forever, without consequence.