No knock warrants are not easier to execute. They are more difficult and dangerous than a day time warrant.
I don't see any reference to the time of day in the legal definition of a 'no knock warrant.'
No Knock Warrant - A no-knock warrant is a warrant issued by a judge which allows law enforcement to enter a premises without knocking or identifying themselves as police first. It is a warrant authorizing officers to enter certain premises to execute a warrant without first knocking or otherwise announcing their presence where circumstances (such as a known risk of serious harm to the officers or the likelihood that evidence of crime will be destroyed) justify such an entry.
The stories cited in the original post describe them as occuring at night.