With effect from June 18 midnight, the Immigration, Customs, Quarantine and Security (ICQS) Bukit Kayu Hitam and Customs, Immigration and Quarantine (CIQ) Sadao have extended their operations to 24 hours.

However, the 24-hour operations are for a three-month trial period and involves only cargo, as well as trucks and heavy vehicles carrying goods. For more information, read here.
We have yet to see if the operations will last in the long run, or whether or not Malaysian tourists who drive into Thailand will be able to benefit from the 24-hour operations.
Currently, tourists will be able to drive into Thailand via the Bukit Kayu Hitam-Sadao border between 6am and 12am Malaysian time (18 hours daily).
For those of you who plan to drive into Thailand, here is the document checklist and procedure.