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To speculate about the sleeping postures of extinct dinosaurs, you 
must know what big extant mammals do first.

It is well known that elephants cannot lie for long periods on their 
sides because there is a danger that they then crush their lungs. 
However, nobody really tested this rather good idea and, until 
recently, it was widely thought that a lying down elephant was a dead 
elephant. So strong was this belief that, as reported in one of zoo 
vet David Taylor's autobiographical works, elephants that reclined in 
order to resist the trials of transportation (i.e. in stubborn 
refusal to their being pushed and pulled toward the back of a 
lorry) were sometimes euthanised. How sad.

We now know that this was perhaps a bit over the top. Elephants can 
recline on their sides for at least a few hours without permanent 
damage, and have been filmed several times (both species) doing so. 
Amazingly, the whole herd all recline together and sleep so heavily 
that even land rovers do not wake them up. 

Giraffes do not sleep deeply standing up, but do spend most of the 
night in this position. Once every couple of hours, they fold their 
legs up underneath their bodies, belly pressed to the ground, and 
actually place their head on the ground too. In this position they 
sleep deeply for about 10 minutes, and then raise their head and 
stand up again. Obviously with the head on the ground they are 
tremendously vulnerable to predators. That giraffes do sleep in this 
way was only proven about 4 years ago by way of infra-red nightime
photography. It would be interesting to see if African natives ever 
refer to giraffes sleeping in this way, as presumably they might just 
have encountered this kind of behaviour before.

With respect to sauropods, I think the best guess is that they did 
sleep deeply in this position (i.e. with the head on the ground), but 
otherwise spent most of the night standing up. Maybe they could 
'sleep' one half of the brain at a time, like shorebirds do.

"No caviar for me thanks, I never did like it that much"