The Fourth Amendment generally requires a search warrant in order to search an individual's property. However, unless the warrant itself indicates otherwise, search warrants generally must be executed during the daytime. A night-time search warrant explicitly allows government agents to enter and search the premises at night. Those with specific warrant problems should contact an attorney.
Fourth Amendment law requires a warrant for the government to conduct a search (unless the search meets the requirements for certain exceptions.) Search warrants must be issued by a neutral judge, based on a finding of probable cause (meaning reasonable belief that the searcher will find evidence of a crime.) Should police search without a warrant or some exception in play, any evidence seized during the search will be inadmissible in court.
A valid search warrant must be precise; it must detail the precise areas that government agents may search, exactly what they're looking for and what types of discovered evidence they may seize. Any special requirements or authorization connected with the warrant, such as "night-time execution," must be explicitly described in the warrant itself.
Search warrants are typically understood to authorize "day-time" searches. Each jurisdiction has its own definition of what constitutes daytime, but most tie it to the idea of regular business hours; one common interpretation is 6 am to 10 pm. A night-time search warrant allows search beyond that jurisdiction's prescribed daytime hours. Note, however, that if a valid search begins during the day and extends beyond the "night" limit, police will not need to get a night-time search warrant in order to continue the search.
To obtain a night-time search warrant, the police officer requesting the warrant must generally present proof to the judge that circumstances justify night-time execution. Generally, judges issue night-time search warrants for two reasons. The first is that daytime execution may compromise officer safety whereas night-time execution would not. The second is that if the search commences in the daytime, there's a significant chance that those on the premises may attempt to destroy the evidence before police can locate it.
Should police have a valid search warrant, any evidence they seize may still be excluded from court if the defendant can demonstrate that police behaved improperly in actually executing the warrant. So, for instance, police executing a daytime warrant in the night-time could find all of the evidence seized in that search inadmissible in court.