Looks like this is what you want:
System.sleep(uint16_t wakeUpPin, uint16_t edgeTriggerMode, long seconds) can be used to put the entire device into a stop mode with wakeup on interrupt or wakeup after specified seconds. In this particular mode, the device shuts network subsystem and puts the microcontroller in a stop mode with configurable wakeup pin and edge triggered interrupt or wakeup after the specified seconds. When the specific interrupt arrives or upon reaching the configured timeout, the device awakens from stop mode. The device will not reset before going into stop mode so all the application variables are preserved after waking up from this mode. The voltage regulator is put in low-power mode. This mode achieves the lowest power consumption while retaining the contents of RAM and registers.
// EXAMPLE USAGE
// Put the device into stop mode with wakeup using RISING edge interrupt on D1 pin or wakeup after 60 seconds whichever comes first
// The device LED will shut off during sleep
I guess you use an unused pin if you only want a time based sleep.