By relying on ambient energy, battery-less devices significantly increase the autonomy of IoT devices, enabling maintenance-free operation in remote locations. However, due to the scarcity of ambient energy, these devices rely on capacitors to buffer energy, and alternate between power-off phases where the device is harvesting energy and computation bursts. In most existing techniques, the device resumes execution only when the capacitor is full. However, we argue that doing so is sub-optimal. Instead, we advocate that waking-up the device sooner may yield better performance since the microcontroller consumes less power when operating at lower voltage. To this extent, we introduce EarlyBird, a technique that automatically computes a fine-tuned wake-up voltage for each resume point. EarlyBird leverages static analysis to determine how much energy is needed before resuming from a given program location, and provides a runtime library to enforce the early wake-up strategy. We evaluated how EarlyBird improves existing checkpointing techniques and results show an increase in the number of benchmarks executed per minute of up to 5.65x.