No they aren't . 8 hours consecutive in a sleeper means exactly that .
Part 395.8 (h) (1) Off-duty. Except for time spent resting in a sleeper berth, a continuous line shall be drawn between the appropriate time markers to record the period(s) of time when the driver is not on duty, is not required to be in readiness to work, or is not under any responsibility for performing work.
(h) (2) Sleeper berth. A continuous line shall be drawn between the appropriate time markers to record the period(s) of time off duty resting in a sleeper berth, as defined in §395.2. (If a non-sleeper berth operation, sleeper berth need not be shown on the grid.)
We know it's done often but it isn't "allowed"