Just to add to what Firefox has said and bring up a point mentioned in a previous thread....if you do anything under certain circumstances in law when there is a chance of a particular outcome, then you may be prosecuted because you were 'reckless' to the outcome occurring.....i.e... have a loaded shotgun in public risking it going off.....drive while tired and wandering over the road (Dangerous Driving) risking an accident.....chucking an object carelessly which causes damage or injury.....lighting a fire in certain places etc.
No one can 'make' you do anything which can lead to a criminal offence....it is both against your human rights and that person can be held personably liable as an accessory....so should a police officer, ranger, stewart, or whatever make you move and you have an accident as a consequence from say drifting off at the wheel....then you have a case against them.....
To reiterate advice already given.....I'd simply say that I'm so tired that I'm a bit dizzy and unsafe to drive and tell them you are going to note their vehicle registration and or take their details and photograph and reserve the right to take a civil case or make a complainst against them if they force you to do an act which may lead to an unlawful outcome or personal injury.