As a night-time person, you are more awake when the sun goes down. Your Intelligence and Perception are improved at night, but dulled during the day. You get a -1 modifier to these two statistics from the hours of 0601 to 1800. You get a +1 modifier to these statistics from the hours of 1801 to 0600 (or 6:00AM to 6:00PM, if you want to look at it that way).