Hello JaylenWang,
Thank you for reaching out Surface Community. When the Surface is in sleep mode it is possible there a many apps still running in the background and when the temperature gets high, the thermometer icon is designed to drop down to prevent overheating. If it gets too hot, it will shut down the tablet instantly and will not turn on until it cools down. It also helps preserve battery and decrease cooling cost or can be useful as a security measure for overheated systems. If it is doing that with regular programs, you can manually shut down or change your power management plan. Go manual on your power management settings and attempt to set a power plan that will work better. It may reduce some of the problems you are encountering.
Regards,
KarenJune_F