Seriously?They are level 3 now, and I use level 3-5 encounters. I have never used something beyond their level+2.
My last session involved Level 1 characters - after a full session the previous time, they had had no time for more than a 5-minute rest, and were getting tired enough that they needed to make Endurance checks or suffer potential negative consequences ... but things are moving too fast for them to rest.
Their encounters - for a party of 4, with no rest - were:
650 xp - 2 L2 Skirmisher, 2 L1 Soldier, 8 L1 Minion
750 xp - 3 L2 Skirmisher, 3 L2 Artillery
725 xp - 1 L5 Brute, 1 L3 Skirmisher, 3 L2 Skirmisher
750 xp - 1 Elite L3 Skirmisher, 2 L3 Soldier, 6 L1 Minion
(550 xp - 1 L5 Obstalce, 2 L4 Soldiers
+625 xp - 1 L4 Controller, 1 L3 Controller, 2 L2 Skirmisher, 6 L1 Minion
+ even worse deeper into the battle)
PC's goal is simply to get past the L5 Obstacle in the final encounter, before the Shaman (L4 Controller) does, without getting caught.
a.) If they stand and fight, they face basically "every remaining encounter at once"
b.) If they hide and sleep, the Shaman gets the McGuffin
c.) They can gain XP for successfuly avoiding enemies, not just defeating them.
The L5 Obstacle challenges them to get past it before the reinforcements arrive (reinforcements arrive staggered) to make the "fight" portion desperate.
The party was allowed the benefits of 5-minute-rests between encounters, and was allowed to level up and regain 2 healing surges between encounter 3 and encounter 4.
That made the day's script:
Level + 2.5
Level + 3
Level + 3
Level + 2.5
Level + 4
All with no extended rest .. plus chances of Surprise, negative terrain, penalties from failed Endurance checks, the works.
The result?
The party made it to the final encounter, got past the L5 Obstacle with all four PC's standing, and got the doors shut to bar the surviving bad guys outside - except for the L4 Controller, who took multiple Opportunity Attacks to get in with them. He got lucky, dropping two of the party members who still had healing surges left to exactly zero hit points, but the other two took him down with scant hit points to spare.
.....
Why am I describing this? Not because I recommend pushing your group that far, but just to point out just how much you're coddling your group if you're afraid to go past Level + 2 and you let them sleep every thirty seconds.