In this paper we present a novel wireless sensor node design, called BodyANT. BodyANT has a miniature outline of 20x10x3mm and reaches almost 5 days of battery operation when continuously transmitting acceleration data at 32 Hz. We evaluate the performance of BodyANT in an extensive set of laboratory and naturalistic experiments over multiple days. Our sensor node implementation showed robust performance for up to eight simultaneously operating nodes. We deployed three BodyANT nodes in a study of daily activities in combination with two further devices, a heart rate monitor, and a GPS receiver and analysed the network performance over multiple recording days. The data loss per sensor node in the experiment was 3.3% on average. We present and discuss detailed results on data losses per user activity. Our results demonstrate the feasibility of using BodyANT for long-term activity monitoring and recognition applications.