The Polar M400 is my first Polar clock. It is relatively large, but comfortable to wear, even at night - because if one emphasizes a sleep analysis. Since OS seems a bit archaic and slightly reminiscent of the old C64, BUT: this is what makes this clock so fascinating! While competitors like watches to log times, crash, or out of air, the Polar running. GPS works very well outdoors, indoors it needs a little longer to get a signal. The comparison with Polar Flow works well, even with the mobile Polar-Alp. So what I lol this fantastic clock give more than 5 stars?