So what is "Dead Time"? Well, this opinion varies from person to person. Some paranormal investigators believe that 3am is the time when paranormal activity is at its peak. These select investigators feel that, in essence, it's the most active time because one theory is that Jesus died at 3pm, so the opposite is 3am. They feel that this particular time, because of this very reason, tends to be the preferred time of spirit activity.
Now personally, I don't really feel the same way. Spirits are active 24 hours a day and I've seen no evidence that 3am is more active than say, 1am or 5am. Ghost hunting is done in the dark more for practical and technical reasons rather than a personal belief about a particuliar time. On the practical side, paranormal activity is more prevalent at night. I mean, you typically don't tell ghost stories at 1pm in the afternoon, do you? On the technical side, darkness is more suited for using cameras, recorders and other instruments, especially IR and FLIR cameras. It's easier to see possible manifestations, apparitions and anomalies at night, that's a given. But nighttime is also better suited for a persons senses: the environment is more quiet.
But nighttime can also play on your senses in a negative or false manner. Since everything is quiet you tend to hear more of the subtle normal sounds of a home or the environment, the sounds that are typically present at all hours. Also, the night is usually cooler. The wood used in houses expands in the heat of the day and constricts at night, causing popping and creeking sounds. The sounds can lead a novice or rookie investigator to believe they are caused by the paranormal. The darkness can play "tricks" on your eyes, fooling you into thinking you see shadows or something that's not really there; this is especially true if investigating in a wooded area.
There are other speculations about "Dead Time". Some believe that 3am is actually the devil's hour. They also believe that 3am is not the time when all spirits are more active, but rather when demons are active. Again, they believe that since Jesus died at 3pm then 3am is the polar opposite. Demons will do what they can to defy Christ and therefore being most active at the polar opposite 3am is what they enjoy. Also, more specifically, 3:33 am is widely believed to be the "Witching Hour". Supposedly, 3:33am is the time when the veil between the two worlds is the thinnest, allowing the spirits to cross over easier.
So, is this all true? Well, it's an opinion. There's nothing to really support these theories other than some personal experiences. But there are just as many personal experiences contradicting this "Dead Time", experiences that occur at all hours. For me, I don't believe in "Dead Time". I think spirits are active at all hours. Sure, nighttime may yield more results for ghost hunters, but this is because our senses are hightend at night.