Board Thread:Warhammer 40k General Discussion/@comment-25618639-20150220210346/@comment-6078851-20150312150129

they did use sleeper ships but they were for pan-galactic scale voyages. short trips through warpspace are relatively safe, and were utelised, but when the distances start to span segmentums of the galaxy that's when an astronomican (or devices creating a similar navigation point) are needed as the majority of long distance warp-journey's ended up lost. 6th edition rulebook quote, "m18 - m23, dark age of technology: mankind realises its destiny amongst the stars, colonising world after world at a rapacious pace. warp space is tamed and the first alien races subjugated. an age of expansion and plenty begins."

so dark age of technology era sounds about right imperialfist, but how old was the previous empire? we know when it effectively ended but we've no idea of when it started. the quote says warpspace was 'tamed' during the dark age of tech, so this suggests that it was known of beforehand, when it was wild and untamed.