These 60-hour shipping warmers are ideal for two-day shipments with cooler nighttime temperatures. Please note that it may take up to 60 minutes for these heat packs to start generating noticeable warmth. Uniheat shipping warmers are specifically designed for small pets, tropical plants, and other temperature-sensitive items. For optimal performance, be sure to open the shipping warmers well in advance of your packing time. This allows ample time to ensure proper activation and reliable warmth during transit.