I suggest that there be an option for taking long rests where you can take an extra 2.5 hours for the rest (total of 10.5 hours), to allow each of the four characters to take a 2.75-hour-ish watch shift and still get in their full 8 hours. Not only is this a sensible practice when traveling in hostile lands, but it will allow a character to be awake when enemies approach. They'll still have to make a perception roll to spot the danger and avoid a surprise round, but the chances are that the'll notice the enemy before they're plunging a sword into their bellies.
The drawback is that longer watches means longer time taken, which could increase the amount of travel time, and it could be a problem for more time-sensitive matters, so there could be a choice whether to speed up and risk being ambushed, or go slower but safer.
I also think that when ambushed during a long rest, it would be sensible to allow an awake character to use a bonus action to shout and awaken everyone else who is asleep, instead of using a full action to manually wake up one person. This is only for normal, long rest sleep, and would not apply to magical sleep. The people who are awoken in this manner are would still be prone and possibly (likely) surprised.