This paper investigates the energy consumption of Environmental Internet of Things (E-IoT) systems using CoAP, RPL, and 6LoWPAN. We focus on Sleepy Networks, a method to save energy in battery-powered nodes where nodes spend most of their time sleeping and have regular communications windows. Our experiments used the Microchip SAM R30 Xplained Pro platform and measured the current and time duration of system activities. This included reading sensors, storing data, and the radio activities controlled by the network protocols. After measuring the power consumption of the various states in detail, we explored the effect of network maintenance on scheduling and energy consumption. The results indicate that reducing sleep energy consumption is fundamentally important. While considering the user requirements and the volume of produced data, the results show that optimising the communication window can reduce energy consumption further. Therefore, we suggest to further reduce the energy consumption of the system, and it is necessary to take a balance between network reliability and energy consumption.