Resting in the wasteland is vital to your survival you must sleep atleast 4 hours in a day or you will suffer 1 level of exhaustion. To rest in the wasteland you must meet one of the following sets of conditions. 

Conditions For Resting 

Resting in a bed in a settlement camp or outpost for 4Hrs allows you to recover upto 50% of your Max Hp, Sleeping for 8Hrs  Alows you to recover 100% and is considered a Long rest if you have Drunk and eaten enough before resting this will remove 1 level of exhaustion. 

Resting in ruined building Tent or similar shelter, with a bed or bedding including a sleeping bag, a bedroll or a Hammock for 4 hours allows you to recover upto
40% of your maximum Hp. with 8Hrs sleep alowing You to recover upto 80% of your maximum  Hp. additionaly sleeping for 8 hours when useing a Sleeping bag and a Bedroll or Hammock, will recover i level of exhaustion providing you have drunk and eaten enough that day. 

Resting near a campfire with bedding  including a sleeping bag, bedroll or hammock for 4Hrs will allow you to recover upto 35%  of your maximum Hp, additionaly sleeping for 8Hrs will allow you to recover upto 70% of your maximum Hp

Resting in ruins, a tent, similar shelter or near a fire without bedding for 4Hrs will allow you to recover upto 20%  of your maximum Hp, additionaly sleeping for 8Hrs will allow you to recover upto 40% of your maximum Hp

if you do not meet the minimum requirements for rest, or you are in imediate danger you cannot rest.