Is it at all possible to have a decent night sleep in a heavy suit of plate armour? What about other types of armour (mail, plate mail, etc.). Also, how much care did they required (cleaning, oiling, etc.)? Lastly, I found minimal information on boiled leather. Were these widespread? How efficient is a suit of boiled leather versus typical medieval weapons (swords, battle axes, arrows, spears, etc.)?
To answer the leather armour question, you see a brief period of hardened leather being used to reinforce maille in the late 13th and early 14th century, but unlike in games and other media this does not appear to have been a 'poor man's armour' that would clad common soldiers. It was a supplement to contemporary knightly armours of maille. It was also used in certain kinds of tournaments in later periods. I don't know of much documentation on how effective it was.
Typically, people used cloth armour instead (called a gambeson, aketon, or jack, depending on the period and construction of the garment), either padded with raw cotton* or made up of many layers of linen. Contemporary accounts mention archers with arrows sticking out of their cloth armour being unhurt, so it appears to have been fairly effective against many types of attacks; I have tried stabbing a 30 layer jack with a sword and cannot achieve significant penetration.
PS: 'plate mail' is not a historical term and is not used by armour historians. If you're referring to combinations of maille and plate (like this ottoman armour: http://upload.wikimedia.org/wikipedia/commons/thumb/5/53/Istanbul.Topkapi067.jpg/220px-Istanbul.Topkapi067.jpg ) then this was used as full battle armour by soldiers in the Near East, Central Asia and South Asia.
Sources: European Armour 1066-1700, Claude Blair
Arms and Armour of the Medieval Knight, Edge and Paddock
*cotton fabric wasn't much of a thing in medieval Europe, but raw cotton could be used as stuffing.
hi! fyi, there have been some questions on leather armour; check 'em out
