Most of their sleeping is very light more what we would consider a very relaxed state. This kind of sleep is easily interrupted and allows them to be aware of their surroundings. Cows also need about an hour of a deeper kind of sleep which they take in short naps of a few minutes each spread over a 24 hour period. Because each individual cow spends so little time in deep sleep most cows in a herd are alert even at night. It s not that easy to sneak up on a herd of cows.
Domesticated cows are relatively calm animals but they are spooked by unexpected events.
Cows still have the characteristics that kept them alive when they were wild animals avoiding predators. They have eyes on the side of their heads so they can see in all directions and ears as big as feet for purposes of detecting possible attackers. One alerted animal easily transfers its concern to the others in the herd. If you live in a town you may not realize how incredibly quiet it can be out in the country at night. You can hear a lone car coming for several minutes, the noise of the engine slowly increasing as it approaches.